QUARTERLY REVIEWS SYNTHETIC GEMSTONES

By E. A. D. WHITE (THE GENERAL ELECTRIC COMPANY LIMITED, HIRST RESEARCH CENTRE,

CENTRAL RESEARCH LABORATORIES, WEMBLEY, ENGLAND)

GEMSTONES may be defined as inorganic compounds, usually single crystals, which show colour effects of great aesthetic appeal. The demand for these materials, and consequently their economic value, depends on several factors, including rarity, durability, crystal perfection, colour, physical properties, and, to a considerable extent, fashion. The history of gemstones has shown that their interest originally derived from naturally faceted stones, which were held in higher esteem than irregular or partly faceted stones such as diamond. As cutting and polishing techniques developed the full beauty of diamond and corundum was appreciated. The present demand for gems for adornment is restricted to comparatively few materials-mainly diamond, emerald, ruby, and sapphire-although many of the less known species are extremely beautiful.

The comparative rarity of most gemstones has imposed a high com- mercial value on them which has provided the main stimulus for their synthesis. Although the ornamental glass beads of ancient Egypt, and the later Assyrian paste gems, indicate early attempts to simulate natural products, systematic efforts were not made until the 19th century. In parallel with this economic stimulus the scientific interest in geology and geochemical processes increased in the same period, but early experiments were severely restricted by the low temperatures and pressures attainable. The development of new refractory metals and ceramics enabled the con- ditions for the natural formation of many mineral species to be reproduced in the laboratory, or, where this was not possible, for alternative methods to be employed. The synthesis of diamond represents the culmination of these efforts, even though large single crystals have yet to be produced.

To the gem dealer, synthetic materials are undesirable since they threaten to reduce the value of natural stones, unless-as for diamond and emerald ~

-the process is complex and expensive. This is true even though the synthetic may be more perfect than the natural material and indistinguish- able from it without the use of specialised test equipment. Thus synthetic ruby is a relatively cheap commodity which is mainly relegated to use for “jewel” bearings, while good-quality natural stones command a high price.

The aesthetic appeal of crystals in general is still appreciated by the modern crystallographer and crystal grower, but new impetus for research into growth theory and techniques has been provided by the increasing need for crystals for technological and scientific purposes. The study of 1 1

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2 QUARTERLY REVIEWS

single crystals has become important for the full assessment of physical properties and for studies in the rapidly advancing field of solid-state physics. In many instances the special attributes of gemstones make them of great scientific interest, although additional properties, such as para- magnetic resonance phenomena, are becoming increasingly important. In particular, the stability and durability of certain gemstones make them useful materials. Stability in this sense includes chemical stability towards heat, moisture, and corrosion ; stability towards abrasion and mechanical damage; and stability with respect to variation in physical properties with temperature and time. The major gem materials are stable in most of these respects and consequently find diverse scientific applications.

The present Review is confined to synthetic crystals and does not in- clude imitation gems, or gem materials of amorphous, polycrystalline or organic nature.

Crystallisation Processes The aims of gem synthesis are two-fold. From the scientific viewpoint it

is desirable to produce large single crystals of good quality; as gems the products should be indistinguishable from natural stones, that is, they should contain imperfections of the same type as natural crystals. The latter aspect has largely been ignored by workers in this field, although in certain instances synthetic stones resemble the natural counterparts very closely. This is usually true when the crystallisation process used closely parallels a natural geochemical process, as for example, in hydrothermal growth.

In the present instance we will be concerned with the production of good-quality crystals which are, perhaps, more perfect than the natural counterparts. However, in one respect the synthetic gemstones cannot compete with natural material; this is in their sheer size and the scale of their growth conditions.

Conditions for the Growth of Large Single Crystals.-Crystal-growing unfortunately has not received the same amount of attention as other aspects of crystallography. No single theory of growth has been postulated which is capable of universal application, and the theory has in fact lagged considerably behind practice. A wealth of empirical information has ac- cumulated over the last few decades, although there are few basic text- books in which the main principles are set 0ut.l The following factors are accepted as being of importance in growth processes.

(1) Rapid growth does not, in general, lead to crystals of good quality. Considerable strain can be introduced into the lattice which may cause gross cracking, mosaic effects, or high dislocation densities. Inclusions, which may be gaseous, liquid, or solid or a combination of any of these,

Buckley, “Crystal Growth,” Wiley & Son, New York, 1951 ; Lawson and Nielson, “Preparation of Single Crystals,” Butterworths, London, 1958 ; “Crystal Growth,” Discuss. Faraday Soc., 1949,5 (published in book form by Butterworths, London, 1959).

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WHITE : SYNTHETIC GEMSTONES 3

are common and compositional changes may be experienced during growth owing to depletion of a constituent from the growth medium.

It is significant that the largest and most perfect synthetic gems have been produced by the relatively slow hydrothermal process. However, in certain cases it is expeditious to use rapid methods if the size and quality of the crystals are suitable for a particular application.

(2) Temperature fluctuations can lead to many of the defects mentioned above. It is essential in most processes that variations should be avoided to achieve complete continuity in the deposition of material on a growing crystal. Ideally, growth should take place under a constant degree of supersaturation or undercooling, but it is not usually practicable to make the necessary measurements under the conditions used. It is preferable that growth should occur at constant temperature in order to avoid lattice strain, but few techniques approach this condition.

Elaborate temperature-control devices are usually an important part of any crystal-growing equipment and may constitute its major expense. This particularly applies to techniques operating in the range from room temperature to 1600". Above this temperature considerable experimental difficulties are involved both in the sensing and the control devices, and a lower degree of control is usually accepted.

(3) Impurities present during crystal growth can lead to any of the defects listed above, depending on their concentration and nature. With the general increase in purity of modern materials it is often unnecessary to use other than Analytical Reagent grades of chemicals as starting materials. For certain scientific uses, however, it is necessary to consider impurities at or below the level of parts per million, and the initial purifica- tion by such techniques as zone melting or ion-exchange chromatography is then essential.

A major source of impurities lies in the container used for growth. Crucible materials must be chemically inert, although it is difficult to avoid slight contamination from this source. Certain techniques eliminate this trouble by using the material to be grown as a support, e.g., the growth of semiconductor materials by the floating-zone technique,2 and the flame- fusion technique (see p. 13).

Most natural gemstones have a relatively high impurity level, which in many cases is directly responsible for the colour ; impurities are often added in gem synthesis for this purpose. It should be noted, however, that minor structural distinctions can exist between natural and synthetic minerals as a result of the great purity of the latter.

(4) As a general rule the size of crystals obtainable increases with the scale of the technique. This is due partly to the larger amounts of material available for growth and partly to the more stable temperature conditions consistent with a large thermal mass. Spontaneous nucleation which

Keck and Golay, Phys. Rev., 1953,89, 1297.

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4 QUARTERLY REVIEWS

would provide crystals competing for growth must, of course, be avoided. ( 5 ) Growth from solution is limited by the availability of material at

the growing surface. In the proximity of a growing face a layer depleted of solute is p re~en t ,~ and growth would stop if fresh material did not be- come available. Similar considerations apply to growth from the melt, where dissipation of the heat of crystallisation in the surrounding liquid is essential if a suitable heat sink is not provided, In both instances it is desirable to assist diffusion by efficient stirring. This is not always practicable (e.g., in the fluxed melt) and the crystals produced are fre- quently dendritic in nature, although externally they may appear to be of good q ~ a l i t y . ~ In the hydrothermal method, thermal circulation provides very effective agitation.

Basic Crystallisation Processes.-The basic crystallisation processes which are used for crystal growth are summarised in Table 1. Each of these

Basic method: growth from

Vapour-phase

Gaseous solution

Aqueous solution

High-temperature solution

Pure melt

Solid state, at high- pressure

TABLE 1. Crystallisation processes. Conditions Technique

Usually controlled pressure Hydrothermal method; high pressure and temp.

Hydrothermal method; < IOO", atm. pressures for water-sol. crystals

Atm. pressure at high temp.

High temp., pressure de- pending on substance

High temp. and pressures

(i) Decomp. at hot wire (von Arkel) (ii) Growth from pure vapour (i) Isothermal

(ii) Temp. gradient (iii) Pneumatolysis

(i) Isothermal (ii) Temp. gradient (regenerative

and circulatory techniques) (iii) Temp. lowering (iv) Precipitation (i) Temp. lowering

(ii) Evaporation of solvent (iii) Compositional changes in melt

(i) Temp. lowering (Bridgman, Stockbarger)

(ii) Crystal pulling (Kyropoulos, Czochrals ki)

(iii) Flame-fusion (Verneuil) (iv) Zone melting (floating zone and

crucible techniques) (i) Sintering

(ii) Hot pressing (iii) Diamond-forming process

has been developed into a number of techniques designed to meet the particular requirements of a material. Several of the older techniques have been further adapted or modified for the growth of new materials, and con- sequently many of these differ only in slight detail. Some of the more important factors concerned with these processes are compared in Table 2.

It is difficult to assess the relative merits of the different techniques as, in many cases, only one is conveniently applicable to a particular substance. Where a choice is necessary, the intended application of the products may

Berg, Proe. Roy. SOC., 1938, A, 164, 79; Bunn, Discuss Faraday Soc., 1949,5 132. Doughty and White, Actu Cryst., in the press.

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be decisive; for instance, if crystal perfection is not essential, as in crystals for gem usage, a rapid method using simple apparatus becomes economic- ally preferable. On the other hand, the chemical and physical character- istics of a material will often largely determine the growth techniques which it is possible to apply. The main factors are as follows.

Dissociation in the molten state is undesirable, but if the components are of similar volatility growth is then still possible. For relatively unstable compounds it is necessary to use a low-temperature method.

For materials of low melting point (< 1000a) direct melting is a conveni- ent method. At higher temperatures (1000-2000") contamination by reaction with the crucible material becomes increasingly troublesome, and the flame-fusion technique is particularly useful provided other factors are favourable. Materials with melting points above 2000" are usually obtained from solution or by hot pressing, although certain specialised but uncon- trolled techniques, such as arc-melting, may be used (e.g., for Sic and MgO). Materials which melt incongruently are usually obtained by solution techniques, although they may be grown from the pure melt if the presence of other phases can be tolerated.

Many borates, silicates, and aluminosilicates form glasses from the melt, even on very slow cooling (e.g., quartz). For them it is essential to use a solution technique.

For some materials reversible structural transitions occur at tempera- tures below the melting point. If these involve only minor changes in the lattice, crystals grown from the melt may remain intact on cooling, but in certain cases (e.g., ZrO,, SO,, and BaTiO,) more profound changes occur which cause the crystals to shatter. It is preferable then to use a technique which permits growth below the transition temperature. When the high-temperature form is required this may sometimes be "quenched in" by rapid cooling, but this is normally undesirable owing to the excessive strains introduced. The hydrothermal technique has produced metastable phases under unexpected conditions (e.g., growth of cristobal- ite from siliceous solutions at pH - 7.0) and often both high- and low- temperature forms can be obtained (e.g., a- and p-eucryptite, a- and p- ~podumene).~

Few minerals are water-soluble at room temperature, but most oxides become appreciably soluble at temperatures above 100". The hydrothermal technique is thus of wide applicability, and is only restricted by hydrolysis of the constituents. Many inorganic compounds behave as good ionic solvents when molten and can consequently be used for crystal growing by techniques similar to those used for aqueous solutions (the fluxed-melt technique). Lead oxide, alkali halides, and other metal halides are particu- larly effective provided that excessive chemical reaction does not occur between the solvent and a constituent of the solute.

ti Barrer and White, J., 1951, 1167; Roy, Roy, and Osborne, J. Arner. Ceram. Soc., 1950,33, 152.

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WHITE : SYNTHETIC GEMSTONES 7

The use of gases as solvents has not been explored for crystal-growing. The related pneumatolytic process does not appear promising in the present form6 and s.uffers from lack of control. The fact that several materials are soluble in gases’ at high pressures suggests that gases could be used as an alternative to water in modified “hydrothermal” equipment.

For materials with high latent heats it may be preferable to use a technique in which the seed crystal is provided with a heat sink in order to avoid the tendency for dendrite formation.

For crystals of low symmetry which have a large thermal-expansion anisotropy, severe cracking may occur when a temperature gradient or temperature-lowering is involved. This may sometimes be overcome by careful design of equipment, or by particularly effective temperature- control. In the flame-fusion technique the use of a subsidiary heater round the growing crystal has been used successfully for fefrite materials.* A low-temperature solution method is preferable in other cases.

Materials of low thermal conductivity also tend towards dendrite formation and slow growth is desirable for them.

Materials which sublime are often conveniently crystallised from the vapour (e.g., ZnS, CdS). In other such cases the decomposition of a volatile compound, or an inert gaseous medium as a diluent or carrier- medium, can be used. Where one constituent is more volatile than another, the composition of the product can be conveniently controlled by external control of the vapour pressure of the more volatile constituent (PbSc, PbTe).

In order to grow large crystals it is essential to control or restrict spontaneous nucleation. This is most conveniently accomplished by the use of a seed crystal; the degree of super-saturation or undercooling is then restricted to values suitable for growth on the seed and is not allowed to reach values at which spontaneous nucleation is possible. Various tech- niques have been devised to allow only one crystallite to develop from a number of nuclei formed spontaneously (e.g., by Verneuil, Czochralski, Bridgman, and Stober).

In high-temperature solution techniques, it is sometimes difficult to provide seed crystals, and nucleation is therefore controlled by a very slow increase in supersaturation so that the nuclei formed intitially act as seeds and restrict further nucleation.

In specialised techniques, factors such as the surface tension, vapour pressure, and viscosity of the liquid can be important, and may necessitate modification of the equipment used.

Highly reactive materials may also introduce complications and necessitate the use of inert atmospheres, special crucible materials, or special safety precautions.

Michel-Levy, Discuss. Faraday Soc., 1949, 5, 325.

Harrison, Research, 1959, 12, 395. ’ Morey and Hesselgesser, Ecun. Geol., 1951,46, 821 ; Morey, ibid., 1957, 52, 225.

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Over-all it should be noted that the hardness, chemical stability, and consequent high melting point associated with most gem materials restrict the number of techniques which can be readily applied. In order to avoid work at very high temperatures solution techniques have been commonly used. These are particularly appropriate as they are often related to natural growth processe~.~ Many minerals have been synthesised from the con- stituent oxides by sintering, but large single crystals of gems have been obtained by relatively few techniques. These are dealt with in detail in the next section.

Synthetic Gem Materials The four main groups of synthetic gem materials are classified here

according to the growth technique used. Materials which are similar or structurally related to those named are included in each section.

Diamond.--(a) Principles. Diamond claims precedence for several reasons. It has received the most attention in the past; it is probably the most difficult to synthesise; and it has most recently been prepared.

TABLE 3. History of diamond synthesis. Hannay, 1880

Moissan, 1893

Crookes, 1909

Parsons, 1919

Gunther, Geselle, and Rebentisch, 1934 General Electric Co. of America, 1955 Allmanna Svenska Elektriska Aktiebolaget, 1955

De Beers Consolidated 7 Mines, 1959

Norton Co., 1960

Hydrocarbon mixture heated with lithium in welded wrought- iron tubes at red heat for several hr. Sugar-carbon and iron heated in a carbon crucible in an elec- tric furnace; melt plunged into molten lead. Repeated Moissan’s experi- ments; also exploded cordite in sealed tubes. Repeated earlier experiments with better equipment. Various experiments carried out, some similar to Moissan’s. Synthesis in special high-pres- sure equipment announced. Synthesis achieved by using press made of “new material,” capable of operation at 3000” and 70,000 kg./cm.2.

Synthesis announced but de- tails of processes not released.

Small diamonds claimed

Minute hard fragments of doubtful composi- tion

Minute diamonds claimed for both methods Diamonds claimed only when iron present “Unidentified frag- ments” produced Diamond crystals positively identified

References: Hannay, Proc. Roy. SOC., 1880, 30, 188, 450. Moissan, Compt. rend., 1893, 116, 218; 1894, 118, 320. Crookes, “Diamonds,” Harper & Brothers, London, 1909. Parsons, Phil. Trans., 1919, A , 220, 67. Gunther, Geselle, and Rebentisch, 2. anorg. Chem., 1943,250,357. General Electric Co. of America,seeBundy, Hall, Strong, and Wentorf, jun., Nature, 1955, 176, 51. Allmanna Svenska Elektriska Aktiebolaget, see Liander, A.S.E.A.J., 1955,97. De Beers Consolidated Mines, “The Times”, London, Nov. 18th, 1959. Norton Co., ibid., April Sth, 1960.

Barrer, Trans. Brit. Ceram. SOC., 1957, 56, 155.

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A brief survey of the attempts at synthesis is given in Table 3. While several of the methods used are interesting and ingenious it is extremely unlikely that success was achieved before the synthesis by the research team of the General Electric Co. of America. Unfortunately, there were no analytical techniques available to establish the structure of the hard materials claimed in many of the experiments, and it is extremely unlikely that the diamond fragments reputed to have been prepared by Hannay are other than natural specimens.1o The recent studies of the carbon phase diagram and the conditions for diamond formation make it certain that previous attempts were in fact unsuccessful.ll

Since the first successful synthesis of diamond, several other organisa- tions have claimed similar success, although detailed information on the techniques used have not been published.

I 1

I f

Diamond I

Yapour I I I

2 0 0 0 O 4000’ Temper at u r e

FIG. 1. Phase diagram for carbon.

The phase diagram for carbon (Fig. 1) makes it clear that both very high temperatures and very high pressures are necessary for the recrystallisation of amorphous carbon or graphite, and the possibility of crystallisation from a suitable solvent under less extreme conditions is attractive.

Crystallisation of diamond by various techniques was attempted by the General Electric Co. of America, with the following results :

(a) By the direct transition, graphite+diamond: no diamond was lo Bannister and Lonsdale, Min. Mag., 1943, 26, 315. l1 Bovenkirk, Bundy, Hall, Strong, and Wentorf, jun., Nature, 1959, 184, 1094.

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obtained, even at 120,000 atm.; increasing pressure slowed down the transformation.

(b) From carbon-oxygen systems: only graphite was formed, but there was some evidence for diamond from nickel and lithium carbonate.

(c) From systems involving carbon as salt-like carbides: only lithium carbide gave evidence of diamond formation.

(d) Various chemical reductions (including Hannay’s method) : CS2, CHCl,, CCl,, etc., when treated with a metal, gave only amorphous carbon.

(e) From systems with carbon dissolved in metals: these systems are most complex: at low pressures (<50O0 atm.) graphite is formed. At 5500-100,000 atm. and 1200-2400”, diamond was produced from various metal-carbon systems.

Only the last technique involving metal-carbon systems showed promise and work was concentrated on this method. It is claimed that the metal need be present in only low concentration and effectively behaves as a catalyst. Some details of the conditions under which diamond is formed have been pub1ished:l’ (i) It is necessary to work in the region of stability for diamond (see Fig. 1). (ii) The metal catalyst must be molten. (iii) The following metals can be used: Cr, Mn, Fe, Co, Ni, Ru, Rh, Pd, Ta, Os, Ir, or Pt. (iv) Seed crystals are not required. (v) Working further into the diamond-stable region gives smaller crystals. (vi) Growth rates are high, approx. 0.1 mm./min. (vii) The transformation takes place across a thin film of catalyst. (viii) Temperature gradients can affect growth due to the change in solubility of carbon in the catalyst. (ix) The type of carbon used affects the products; commercial grades of graphite are preferred. (x) The process is not simple, for graphite is sometimes formed instead of diamond. (xi) The diamonds produced can contain inclusions, particularly of the catalyst. (xii) The crystal habit varies with the temperature of transforma- tion: cubes are formed at the lowest temperatures; mixed cubes, cubo- octahedra, and dodecahedra at intermediate temperatures ; and octahedra at the highest temperatures. No tetrahedra have been obtained. (xiii) The colour also varies with temperature. The products at the lowest tempera- tures are black, changing to dark green, light green, yellow, and finally to “white” at the highest temperatures.

A photograph of synthetic diamond crystals prepared by these methods is reproduced in Plate 1. It is interesting to compare these recent products with earlier photographs where the synthetic diamonds appeared to be irregular in shape, dark, and opaque.

An X-ray study of synthetic diamonds from two sources (G.E. of America and A.S.E.A., Sweden) has been carried out by Lonsdale, Milledge, and Nave.12 A distinguishing feature of the G.E. diamond was that inclusions of nickel were identified in the crystals. The crystals from A.S.E.A. did not show the presence of a metallic impurity, which suggests

la Lonsdale, Milledge, and Nave, Min. Mug., 1959, 32, 185.

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WHITE SYNTHETIC GEMSTONES 11

that the technique used differs in at least some detail from the General Electric CO.% process. As yet, no useful details have been released of the de Beer or the Norton process.

(b) Apparatus. It is clear that early attempts at diamond synthesis failed because of the lack of equipment capable of operating in the required pressure-temperature region. The recent success may thus be partly attributed to the superiority of modern materials and techniques.

The equipment used is basically similar to that used in the conimercial process of hot pressing. The problems associated with diamond formation are: (a ) to find some way to contain a reasonable volume of specimen at high pressures; (b) to apply heat in a controllable manner; and (c) since most materials flow under the pressures and temperatures required, to find a method of transmitting the pressure without failure of the trans- mitting medium. The success of the General Electric Co.’s equipment apparently depends on two things; the “belt” which is described13 as a pre-stressed metal toroid made of successive rings shrunk on to one another; and the use of pyrophyllite as a gasket material-pyrophyllite possesses the useful property that the melting point increases with increasing pressure.

Oxygen

Hydrogen

FIG. 2. Diagrammatic representation of crystal-growing techniques: ( A ) diamond- forming apparatus; and (B) Verneuil furnace.

Equipment similar to that used in the General Electric Co.’s process is shown in Fig. 2A. The pressure is applied by a large-capacity hydraulic press of conventional design, with tapered pistons of special composition, which are brought into opposition within the “belt”. Pyrophyllite gaskets

l3 Hall, Rev. Sci. Instr., 1960, 31, 125.

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12 QUARTERLY REVIEWS

separate the pistons from the belt and provide sealing. The high tempera- tures are obtained by resistance heating, a graphite cylinder being used to contain the specimen.

A second type of apparatus14 has been used which is capable of operation under similar conditions. In this, four conventional hydraulic rams are joined to the apices of a tetrahedral framework converging towards the centre. The rams drive four anvils which are so designed to provide a tetrahedral space in the centre when they are in contact. The specimen is contained in a tetrahedron of pyrophyllite which is rather larger than the central space and acts both as a gasket material and as the pressure-trans- mitting medium. The heating element is a graphite tube as before.

Equipment of this type is in use in several laboratories in America. It has the advantage of being relatively cheap, and simple to operate. The pressures attainable are at present restricted to about lo6 lb./sq. in., but it is probably capable of being scaled up to give higher pressures, although the volume available for sample processing would not necessarily be in- creased. By using commercially available 50-ton rams at a working pressure of 4 x lo5 lb./sq. in., samples of 1-2 grams can be treated.

(c) Products. The variation of habit and colour with processing temperature has been described above. The crystals obtained are usually less than 0.5 mm. in any one direction and so cannot compete with natural crystals as gemstones. For industrial uses synthetic diamond is claimed to be as good as, and in some cases, superior to, natural material. Any slight advantage for cutting purposes presumably arises from the geometrical form of the crystals, which would provide more robust edges at the working surface. There has been no suggestion that the synthetic crystals are intrinsically harder than natural stones.

The evident complexity of the growth process, coupled with the facts that it is not completely reproducible and that growth on seed crystals is not practicable, suggests that the process will require considerable develop- ment before large single crystals can be grown.

Boron nitride occurs normally as a hexagonal structure closely related to graphite. A cubic form having the diamond structure has been prepared with the General Electric Co.’s diamond-forming apparatus ; this has been named borazon. The material is reputed to be slightly harder than diamond,15 but no evidence for this has been published, nor have details of the preparation been released.

Other materials of interest which have been prepared by dry high- pressure methods include garnet,16 and a dense form of silica which has been named Coesite,17 structural details for which have been published.18

(d) Other Materials Prepared by the Diamond Process.

l4 Hall, Rev. Sci. Instr., 1958, 29, 267. l5 Grenville-Wells, Gemmologist, 1957, 26, 76. l6 Wentorf, jun., Amer. J . Sci., 1956, 254, 413. l7 Coes, Science, 1953, 118, 131.

Ramsdell, Amer. Min., 1955, 40, 975.

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Corundum.-Corundum, in the form of ruby crystals, was the first gemstone to be synthesised in a satisfactory manner (see Table 4). Al- though strictly not synthetic material, the limited success of the recon- structed stones (“Geneva rubies”) foreshadowed the later success of Verneuil in providing large crystals of comparatively good quality.

TABLE 4. History of corundum synthesis. Gaudin, ca. 1869

FrCmy and Feil, 1877

-CU. 1885

Verneuil, 1890 onwards

Paris

Alum and potassium chromate mixture covered with lamp- black and heated in a fire-clay crucible. A1,0, dissolved in PbO in an earthenware crucible and heat- ed for 20 days. “Reconstructed” stones: natural stone fragments fused in an oxy-hydrogen flame. The red colour was revived with potassium dichromate. Developed flame-fusion tech- nique; worked originally with Fremy. A pupil of Verneuil; in attempts to colour corundum with cobalt, magnesia was added and spinel produced.

Small flakes of corun- dum produced

Thin plates produced

“Geneva rubies”

Large single crystals produced

Crystals of spinel pro- duced

References: Gaudin, Compt. rend., 1869, 69, 1343. Frkmy and Feil, see Herbert Smith, “Gemstones,” Methuen, London, 1958. Verneuil, Compt., rend. 1902, 135, 791 ; Ann. Chim. (France), 1904,3,20. Paris, see Smith, op. cit.

The Flamefusion Process.-(a) Apparatus. The apparatus commonly used for the growth of corundum is shown in Fig. 2B and Plate 2. It is still used today in a form which is remarkably similar to that originally devised by Verneuil. It consists of a hopper containing free-flowing alumina powder, the flow of which is restricted by fine mesh gauze. Small quantities of powder are released, by mechanically tapping the top of the hopper, into the oxygen supply, which carries it through the central tube of an oxy- hydrogen burner; it is melted in passing through the flame and falls on to the molten surface of the growing crystal. The process is thus similar to the growth from the melt by the “pulling” technique, but operated in an in- verted position with the melt continually replenished.

The starting process, as devised by Verneuil, is similar to the Czochral- ski or Bridgman technique for restricting nucleation. A small pyramid of unfused powder is built up initially by lowering the flame temperature. The tip of this cone is fused by increasing the gas flow, and as more powder is added, the gas flows are progressively increased. The crystal formed takes up a characteristic shape known as a “boule” (Verneuil’s early crystals were, in fact, globular and the name was more appropriate than it is today).

Flame-fusion equipment based on the original design is now in opera- tion in most of the larger countries of the world. A number of modifica-

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14 QUARTERLY REVIEWS

tions have consequently been introduced, many of which are improvements over the original conception. These are as follows:

(i) An intermittent powder feed can lead to the striations in coloured corundum crystals, and a positive, regular feed has been sought. A success- ful compromise has been the use of electromagnetic vibrators in con- junction with the original type of hopper. Subsidiary vibrators at points in the oxygen-supply pipe have been used to prevent powder collecting on the walls of the tube.

(ii) Originally water-cooled jets were used with a simple oxy-hydrogen burner. The modern trend is to use two separate oxygen supplies in three concentric tubes. The central oxygen pipe of this “tricone” burner carries the powder feed. This arrangement allows a greater variation of the flame- shape and temperature without affecting the powder supply. The use of modern heat-resisting alloys such as nichrome, stainless steel, nimonic, etc., makes water-cooling unnecessary.

(iii) Efficient flow-regulators and gas flow-meters are desirable to avoid temperature-fluctuations. At least one furnace has been produced in which the gas flows are regulated by using a total-radiation pyrometer to detect variations within the furnace enclosure.1g

(iv) Alumina cement bricks are normally employed to maintain a high temperature round the growing crystal. For substances of lower melting- point than alumina it has been found effective to use a subsidiary electric furnace to avoid large temperature-gradients and particularly thermal shock when the growth is finished.

(v) Although with most commercial furnaces the lowering gear is operated manually, it is advantageous to use a mechanical lowering device in order to keep the growing surface in the same position relative to the flame. A smaller type of Verneuil furnace, known as a rod-furnace, employs a photoelectric device to achieve constant lowering.2o A further modification of a rod-type furnace is capable of producing six rods simultaneously. 21

(vi) The use of seed crystals is preferred to the original nucleation method, since this provides controlled orientation of the boule and is more certain of producing single crystals. The seeds are usually small pieces of rod.

(vii) The rotation of the seed crystal about a vertical axis offers no particular advantage, but a development by the Linde Corp. of America has considerably increased the scope of the flame-fusion technique.22 A long seed-rod is held and rotated horizontally in the furnace. A complete circle of molten material is formed on the rod, which consequently builds up into a thin disc. The speed of rotation is adjusted so as to stabilise the molten surface which might otherwise collapse on to the central rod.

I @ M.I.T. Tech. Report No. 144, 1959, p. 15. 2o General Electric Co. Ltd., B. P. 633,118. 21 General Electric Co. Ltd., B. P. 798,818. 22 Linde Corpn., B. P., 809,011.

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By reciprocating the rotating seed rod along its axis much thicker discs can be produced, The obvious advantage of this procedure (apart from the size of the crystal produced) is that the material is grown under virtually isothermal conditions and is likely to be less strained than the normal type of boule. In growing coloured crystals it is desirable to use a seed rod of the same colour as the finished boule.

The original success of the Verneuil technique is undoubtedly associated with the fact that alumina can be prepared in a highly free-flowing state. It is essential for successful growth to obtain the powder in such a condition that even distribution occurs in the gas flow. Fluctuations in the powder feed due to momentary hold-up on the tube surfaces are exceedingly detrimental to successful growth, and sudden “bursts” of powder can cause the growing boule to collapse, or produce inclusions of unfused powder. The feed powder is obtained by calcination of high-purity ammonium alum at temperatures near 1050”. The alum crystals initially dissolve in their water of crystallisation; the solution dries and decomposes to form a soft “meringue” of y-alumina. This is readily broken down by sieving or tumbling to give a light, free-flowing powder. Care must be taken neither to overfire the powder, nor to grind or mill it after firing, as the flow characteristics are easily impaired.

Careful attention must be given to the design of the oxy-hydrogen burner. A reasonably large central-zone in the flame is desirable, which is capable of keeping the growing surface of the crystal molten. A larger burner is not sufficient in itself for the growth of larger crystals, while a simple increase in the rate of gas flow can lead to a smaller hot zone with a cold central region. The “tricone” burner described above is most versatile and allows a large hot-zone to be obtained by the introduction of oxygen relatively slowly.

Turbulence and instability in the gas flow must be avoided. The crystaI grows in a metastable condition; the top surface of the boule shields the lower part of the crystal from the flame and prevents it from melting. Slight fluctuations in the direction of the flame can cause the sides of the crystal to melt away with immediate collapse of the whole boule.

Specific directions for burner design cannot be readily formulated since these depend on many factors, including the nature of the material to be grown, the size of crystal required, the fuel and oxidant gases.

The design of the refractory brick enclosure is important, since it must avoid too large a heat loss, which would lead to a reduced size of boule, or too great a heat retention, which may cause the boule to collapse during later stages of growth. The material and wall thickness used will thus be important, but the internal diameter is probably the most critical factor. The position of the growing boule in relation to the burner is also important and is usually readily determined for given gas flows by trial-and-error.

The boule is usually observed through a small slot cut in the refractory

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bricks, and this should be as small as possible to avoid modification of the shape of the boule.

(b) Products. Corundum crystals can be obtained as long rods measuring up to 450 mm. in length by 1 - 4 mm. in diameter. The cross- section is normally round, but if grown with the c-axis perpendicular to the direction of growth the rod has flattened sides. At orientations near to this, the flattened sides show steps, the number of which increases with the departure of the c-axis from the 90” position, until an apparently smooth rounded surface is observed. Rods of larger diameter normally crack along their length on cooling unless special precautions are taken.

Boules can be grown by the normal technique up to -30 mm. in diameter. Flattened sides are frequently observed for particular growth orientations. The length of a boule can be up to 150 mm.

Boules, as grown normally, split along their length on cooling; the plane of splitting usually contains the c-axis of the crystal, while the flatness of the split surface is taken as an indication of the perfection of the boule. The half-boules obtained in this way may be cut or lapped without further splitting. The splitting apparently relieves the strain introduced in the crystal growth and during the subsequent rapid cooling.

Crystals which remain intact when cold may be heat-treated to 1900” to relieve the and when “annealed” in this manner can be sub- sequently cut without splitting.

(i) Spinel. Second in im- portance to corundum is the production of spinel by flame-fusion from alumina-magnesia mixtures. The ideal formula is MgAI2O4, but in fact the structure can tolerate a considerable deficiency of Mg2+, and mixtures of composition approximately Mg0,1-5Al2O3 are preferred for growth. The boules produced resemble those of corundum in appearance, although orientational “flattening” of the sides is more marked, and orthogonal cleavage or parting is observed, rather than rhombohedra1 cleavage as with corundum.

The main differences from corundum lie in the colours which can be achieved by various additives, and in the comparative softness of the material.

(ii) Rutile and strontium titanate. Natural rutile occurs as dark needle- like crystals, often as inclusions in other crystals (e.g., “rutilated” quartz), and as such has no gemmological value. Synthetic crystals of pure titania can be grown by flame-fusion equipment,24 modified for low-melting compounds. The crystals as grown are usually blackened, owing to reduc- tion of Some Ti4f to Tiw, but when heated in oxygen at -1 500” the crystals become almost colourless. Addition of gallium oxide or alumina to the powder improves the c01our,~~ but the product never approaches the

(c) Other crystals grown by jlame-fusion.

23 Linde Corpn., B. P., 595,445. 24 Zerfoss, Stokes, and Moore, J. Chern. Phys., 1948, 16, 1166. 25 Linde Corpn., U.S.P. 2,756,157.

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PLATE 1. Synthetic diamond crystals grown by the General Electric Co. of America (reproduced by kind permission of that Company).

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PLATE 2. Verneuil furnaces for the production of synthetic coviindum crystals.

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PLATE 3. Synthetic star ruby and sapphires (largest stone approx. 8 mm. in diameter).

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PLATE 4. Synthetic quartz crystal grown by the temperature-gradient hydrothermal process.

PLATE 5. Yttrium iron garnet crystals grown from solution in lead oxide (largest crystal approx. I em. in diameter).

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“blue-white” colour of diamond. The high refractive indices and dispersion give considerable “fire” to a cut stone, but the birefringence can be readily detected.

Strontium titanate has no natural counterpart but is becoming in- creasingly important both as a gem material and as an optical medium. The melting point is near that for corundum and it can be grown under similar conditions.26 It is normally obtained in the darkened, reduced condition and is reoxidised by heating in oxygen. The product is “whiter” than rutile and exhibits similar “fire”, but has the advantage of being an isotropic medium, and therefore closely resembles diamond.

Both materials can be coloured by suitable additives to give stones of various colours. The raw materials used must be of very high purity for the production of “white” stones.

Coloured corundum boules were grown by Verneuil early in the present century. Although the size of boule obtainable has increased, the range of colours available is still limited. The production of spinel was accidentally accomplished in an effort to colour corundum with cobalt, which gives a very patchy effect in corundum but gives a beautiful intense blue in spinel. A great variety of coloured spinels were grown by the German gem industry shortly before World War 11, and are still produced on a limited scale in several European countries. Table 5 lists a number of colouring agents for corundum and

(d) Colouring of corundum and spinel.

Additive Ti V

Cr

Mn Fe c o Ni c u Ti + Fe Ni + Fe Ni + Co Ni + Fe + Cr Mn + Ti Cr + Fe + Ti v + co Mn + Cr + Ti Ni + Fe + Ti

TABLE 5 . Coloration of corundum and spinel. Colour in corundum Colour in spinel

Blue (“sapphire”) Blue-green daylight; pink tungsten light (“alexandrite”) Red (“ruby”) Green or red (depending on

Mg concn. and flame-temp- erature)

Pink Green-yellow Pink

Irregular blue patches Royal blue Yellow (“topaz”) Cambridge blue Blue-green Blue Sapphire-blue Topaz-yellow Deep yellow Orange Orange Amethyst Green Garnet-red Pistachio-green

spinel; these are usually added to the alum before firing at concentrations of -1 % by weight, depending on the depth of colour required.

The introduction of colouring additives into the corundum lattice is limited to ions which are available as or can adopt the tervalent condition,

26 Linde Corpn., U.S.P. 2,628,156.

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which can be incorporated with a second additive to give a mean valency of 3, and which can fit conveniently into the lattice. Thus titanium can be incorporated in the reduced condition, Ti3+, and gives the deep blue colour typical of this state. The addition of both iron and titanium modifies the colour, giving less red in the transmission spectrum, presumably because of the possibility of Ti4+Fe2+ as well as Ti3+Fe3+. It is difficult to incorporate iron alone in corundum, partly because Fe20, is volatile and is lost during growth, but also because of dissociation to Fe2+ which is not readily incorporated in the lattice.

The red colour produced on addition of chromium has been explained by the fact that the ion is too large for the available lattice site at room temperature and is consequently squeezed into an unusual energy state.26 This is supported by the fact that the typical green colour associated with Cr3+ compounds appears when ruby is heated to -200”.

The greater variety of colours attainable in spinel is due to the two sites of different valency requirements available for substituent ions. In addition, the spinel lattice is more flexible in tolerating vacant sites and allowing the distribution of similar ions on both types of site. It is interesting that both the red and the green colour of chromium are displayed in crystals grown under different conditions.

Most colour effects are produced by additive concentrations near 1 ”/:, but for volatile additives (e.g., chromium) it is often necessary to add a considerable excess to provide a reasonable depth of colour. The growth of ruby crystals containing 1 mol. % of residual Cr203 is difficult, although a continuous range of solid solutions can be formed between the end members by sintering. The colour change from red to green is reported2’ to occur at the composition Cr,. osAl,. 9203.

“Star” effects in corundum can be obtained by the introduction of 4 . 3 % of titania into the powder used; the latter presumably enters the lattice as Ti% but on heat treatment in oxygen at -1500” reoxidation takes place with the formation of minute crystallites of rutile, within the corun- dum lattice, which are arranged in the three equivaleni directions per- pendicular to the c-axis. Diffraction effects due to these crystallites can be observed in suitably oriented cabouchon-cut stones and appear as a cross with six rays (Plate 3). This “rutilation” is normaIly restricted to a thin layer near the surface of the stone.

Star stones of various colours have been produced which closely resemble natural star ruby or star sapphire. The star effect is usually more perfect than in natural stones.

The main virtues of corundum are its relative hardness, resistance to corrosion or oxidation, low coefficient of friction for contact with metals, and the fact that it can be grown or fabricated in a variety of shapes. It has been produced on a large scale in Switzerland for many years and is used principally for jewel

(e) Uses of materials grown by j?ame-fusion.

27 Orgel, Nature, 1957, 179, 1348.

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bearings in watches and instruments. The scientific uses are varied and it is becoming increasingly important as a medium where heat-resistance, corrosion-resistance, infrared transmission, or hardness is an important factor. Its use as a maser material for low-noise amplifiers when doped with a paramagnetic ion (e.g., Cr) is now well established.28 Surprisingly, it is not used to a large extent in ornamental cut stones, although the colours of natural ruby can be closely matched.

Hydrothermal Syntheses.- Quartz. Perhaps the most successful syn- thesis of a gem material is the hydrothermal production, on a commercial scale, of quartz crystals. Small crystals of quartz were first produced in early experiments by S c h a f h a ~ t l . ~ ~ It has commonly occurred in the hydrothermal study of numerous silicate and aluminosilicate systems, but no serious attempt was made to produce large crystals until World War 11, when available sources of large, good-quality natural crystals were threatened.

Several techniques for synthesis were developed, ranging from the devitrification of amorphous silica under isothermal conditions, to the use of a complex double-autoclave which required rocking to promote circulation. In most successful methods, which have been adopted both in this country and in the U.S.A., a single container is heated at the base to promote a temperature gradient along its length (Fig. 3). Solution of the

FIG. 3. Diagrammatic representation of autoclave and furnace for hydrothermal growth.

nutrient in sodium carbonate solution takes place at the base of the auto- clave ; the solution rises and in cooling becomes supersaturated, depositing quartz on the seed crystals suspended near the top of the vessel. Full details of the conditions for growth have been published.30 The nutrient material may be fused quartz of high purity; or if the solution is modified, less pure materials such as granite chippings or various siliceous rocks suffice.

Crystals up to 1.0 kg. have been grown, although economically it is 28 Weber, Rev. Mod. Phys., 1959, 31, 681. 29 Schafhautl, Gelehrte Anzeigen, Akad. Wiss. Munchen, 1845, 20, 578. 30 Brown, Kell, and Thomas, Min. Mag., 1952, 29, 858; Laudise, J. Amer. Chem.

SOC., 1959, 81, 562.

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preferable to grow a larger number of moderately sized crystals which require less fabrication for particular applications. The crystals produced are purer than natural crystals and are free from gross imperfections.

Colouring of quartz grown hydrothermally has been accomplished but is less controllable and more limited than that of corundum.31

Apparatus. The types of autoclave used for the growth of quartz vary in detail, but all consist essentially of a substantial metal vessel capable of withstanding internal pressures of 1000 atm. at temperatures up to 500". For low-temperature work mild steel is satisfactory, but stainless-steel or nickel-chromium alloys are preferable for higher temperatures where corrosion becomes troublesome. The most important feature of the vessels is the method of sealing. At low pressures and temperatures a soft-metal gasket is satisfactory, but under severe conditions it is necessary to use more positive sealing techniques. The Bridgman seal,32 embodying the unsup- ported-area principle, has been used satisfactorily at pressures of -1 0,0oO atm. This method has the advantage of being self-sealing, i.e., the seal becomes more positive with increasing internal pressure.

The furnaces used are of any conventional design that is capable of sustaining the necessary temperature gradient along the length of the autoclave. A particularly simple, yet successful system uses a hot plate for both heating and support of the autoclave, with thermal insulation by vermiculite. O

The circulation, and consequently the temperature gradient, may be regulated by the use of internal baffles in the autoclave.33 This in turn provides control of the growth rates. The seed crystals may be suspended at the top of the autoclave by fine platinum wires, or they may be fixed in a metal frame in such a manner that growth in the fastest direction is not restricted.

The pressure is normally controlled by the temperature and the initial degree of filling of the autoclave, although small-scale equipment involving externally applied pressure has been The pressure may either be measured directly by a pressure gauge or transducer, or may be calculated approximately from published tables for water.35 In general, pressure is found to be a less important variable than temperature, and in many cases an accurate knowledge of its absolute value is not needed.

The fastest directions of growth for a particular material may conveniently be determined by the growth on a seed sphere. The results for quartz indicated that the basal plane provided the optimum direction of growth consistent with good quality and the maximum utilisa- tion of the crystal for piezoelectric oscillator fabrication. The seed plates thicken and finally adopt bipyramidal form, with major and minor

Product.

31 Brown and Thomas, J. Phys. Chem. Solids, in the press. 32 See Paul and Warschauer, Rev. Sci. Instr., 1957, 28, 62. 33 Walker, Znd. Eng. Chem., 1954, 46, 1670. 34 Tuttle, Amer. J. Sci., 1948, 246, 628. 35 Holser and Kennedy, Amer. J. Sci., 1958,256, 744; 1959,257, 71.

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rhombohedra1 faces. Normally growth is stopped before capping is complete and the pyramids are truncated by planes, approximating to the basal plane, which have a rough “cobbled” appearance (Plate 4). Little growth occurs in the plane of the seed crystal, although some development of the prism faces is apparent.

Small crystals of corundum have frequently been observed in the hydrothermal studies of alumina and aluminosilicate systems. Because of the comparative success of the flame-fusion growth of corun- dum, no effort was made to grow larger crystals hydrothermally until recently. The potentially higher quality of hydrothermal crystals was of interest both for scientific purposes and because the crystals were not likely to show defects characteristic of the flame-fusion products. The liquid inclusions sometimes observed in hydrothermally grown crystals are, in fact, similar in appearance to inclusions in natural crystals.

The growth of corundum presents several problems which were not encountered for quartz. First, growth at higher temperatures and pressures is necessary both to avoid the stability region of diaspore and to achieve a reasonable solubility of alumina. Apparatus of more robust design is therefore required. Corrosion has been overcome by the use of platinum- lined vessels.

The growth of ruby crystals has been successfully achieved by at least two organisations in the U.S.A.,36 one of these being solely concerned with the production of gem materials. Growth is much slower than is obtainable for quartz, and only small crystals (-1 cm.) are at present available.

Good-quality emerald crystals are the most valuable of the gem materials. Scientifically, it is of little interest except as a source of beryllium, but the possibility of its use as a maser material has been in~estigated.~’ Attempts at the production of emerald were in progress as early as 1848 (Table 6). The first commercial material appeared shortly before World War 11, but the method of production was not divulged and the production of “Igmeralds” was not recommenced after the war. The highly successful Chatham emerald crystals are now produced on a moderately large scale, and nearly perfect crystals have been grown. Full details of the process have not been released but it is evident that it is not controlled as closely as quartz production; the product consists of clusters of crystals, which may have formed by spontaneous nucleation, many of them containing numerous solid and liquid inclusions. The best crystals produced closely resemble natural emerald, both in colour and in the nature of the inclusions present.

Several other minerals of gemmological interest have been prepared hydrothermally although no effort has been made to grow

36 Laudise and Ballman, J. Amer. Chem. SOC., 1958,80,2655; Alexander, Gemmologist, 1959, 28,201.

37 Geusic, Peter, and Schulz-du bois, Bell System Tech. J., 1959, 38, 291.

Corundum.

EmeraEd.

Other Materials.

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large crystals in a controlled manner. Tourmaline,S8 zircon,39 and several garnets 40 have been identified, as well as other crystals classified as minor gemstones.

The versatility of the hydrothermal technique suggests that many other gems of interest could be prepared which present difficulties by other met hods.

Ebelman, 1848

Hautefeuille and Perrey, 1888

Nacken, 1912 Nacken, 1928

Jaeger and Espig, 1934

Chatham, 1935

Wyart and Scavnicar , 1957

TABLE 6. History of emerald synthesis. Heated powdered emerald with boric acid. Heated oxide constituents under lithium molybdate in a platinum crucible to dull redness for 24 hr., then at 800" for 15 days. Addition of chromium oxide gave green crystals. Synthesised emerald and other minerals. Synthesised emerald hydrothermally, using weakly alkaline solutions. Crystals 1 cm. long and 2-3 mm. in diameter obtained at 370-400" (several days). Produced "Igmeralds" by an undivulged method. Druses of crystals up to long obtained. Hydrothermal process used with crushed beryl as feed material. Process reputed to take up to 12 months per run. Hydrothermal synthesis of beryl from Alz03, SO2, and BeCO, at 400-1500 atm. and 600". Addition of chromium oxide gave green crystals.

Small crystals produced Small beryi crystal pro- duced

Emeralds up to 1 ct. obtained

Emerald of 1014 ct. obtained

Transparent crystals obtained

References: Ebelman, Ann. Chim. Phys., 1848, 22, 213. Hautefeuille and Perrey, Compt. rend., 1888, 106, 1889; 1888, 107, 786. Nacken, see Van Praagh, Geof. Mug., 1947, 84, 98. Jaeger and Espig, Deut. Gofdschmiede Z., 1935, 38, 347. Chatham, see Rogers and Sperisen, Amer. Mia , 1942, 27, 762. Wyart and Scavnicar, Bull. SOC. franc. Min. Cryst., 1957, 80, 3956.

Garnet.-The garnet family is complex, consisting of several minerals of specific composition as well as complex solid solutions. The gemmological interest derives from the very great colour range, which includes ruby-red, emerald-green and black. All specimens possess the well-defined cubic garnet structure, with a composition usually near A2f3B3+t(Si04)3 where A = calcium, magnesium, iron, or manganese, and B = aluminium, iron, or chromium. Some replacement of A and B ions by ions of different valency is possible. The synthesis of certain species has been achieved by sintering, hot pressing, and hydrothermal means, but only small crystals have been obtained by these methods. Attention is given below to the technique which has produced large crystals, although not of a naturally occurring garnet species.

38 Frondel and Collette, Amer. Min., 1957, 42, 754. 3n Frondel and Collette, Amer. Min., 1957, 42, 759. 40 Christophe-Michel-Levy, Bull. Soc. frunG. Min. Cryst., 1956, 79, 124; Jagitsch,

Arkiv Kemi, 1956, 9, 319.

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WHITE : SYNTHETIC GEMSTONES 23

The popularity of garnet as a gemstone has waned since the Victorian era. Industrially, the poorer-quality crystals have been used as fine abrasives, but scientifically the garnet family was of little interest until the discovery of the magnetic garnets.41 Yttrium iron garnet (usually ab- breviated to YIG) has the composition Y3Fe,012, where iron fills the B sites as well as those normally occupied by silicon. The occupation of three types of lattice site by two types of paramagnetic ion gives rise to the interesting ferrimagnetic properties, and both single crystals and poly- crystalline ceramics are useful as low-loss magnetic materials for microwave applications. It has the distinction of being one of the first transparent magnetic materials to be discovered, allowing direct study of magnetic domains by optical t echn iq~es .~~

The Growth of Garnet Crystals by the Fluxed-melt Method.-Yttrium iron garnet melts incongruently and is preferably grown by a solution technique, although it has been prepared by flame-fusion under special

FIG. 4. Diagrammatic representation of apparatus for (A) the fluxed-melt method and

condition^.^^ More commonly crystals have been grown from solution in lead oxide by a technique devised by Nielsen (Fig. 4A).44 A mixture of composition near Y 203,1 3Fe20,, 15.4Pb0, is heated in a platinum crucible until solution is complete. Cooling at 1-2"c/h. initially produces crystal- lisation of ferroplumbite Pb0,6Fe2O,; this phase acts as a scavenger for available nucleation sites in the melt, restricting nucleation of garnet at a later stage with the consequent production of fewer and larger crystals

41 Bertaut and Forrat, Compt. rend., 1956, 242, 382; Geller and Gilleo, Acta Cryst.,

48 Frey, Siemens Z., 1959, 33, 577. 43 Rudness and Kebler, J. Amer. Ceram. SOC., 1960, 43, 17. 44Nielsen,J. Appl. Phys., 1958, 29, 390; Nielsen and Dearborn, J. Phys. Chem,

(B) vapour-phase growth.

1957, 10, 239.

Solids, 1958, 5, 202.

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24 QUARTERLY REVIEWS

than are normally obtained from solution. As ferroplumbite crystallises the composition of the melt changes gradually until the garnet field is reached. In a modification of this technique a mixture of lead fluoride and monoxide is used as the flux, giving larger crystals of better quality.45

Products.-The garnet crystals grown by the lead oxide method appear as elongated eicosatetrahedra; they are black, with a high lustre (Plate 5). In thin section the crystals are transparent and dark olive-green. Although often externally perfect in appearance, they may contain gross inclusions of solidified flux and exhibit a skeletal structure which is clearly dendritic in n a t ~ r e . ~ Crystals grown from the mixed oxide-fluoride melt are usually dodecahedra1 in habit and closely resemble the natural garnet form, melinite. The surfaces appear to be etched if cooled to room temperature in the presence of the melt, but are highly polished if the flux is poured off before solidification.

Miscellaneous Gem Materials.-A number of crystal species which belong mainly to the minor gem category have been grown for various scientific or industrial purposes, or have been synthesised in the study of various chemical systems. These materials are listed in Table 7. In general,

Material Blende Zincite Fluorite Feldspar

Hauynite Sodalite Noselite Cancrinite Jadeite Pollucite Spodumene W illemite Zircon Apatite Tourmaline

TABLE 7. The preparation of miscellaneous gem materials, Formula Method of prep.

ZnS Vapour-phase ; hydrothermal ZnO Hydrothermal ; vapour-phase CaF, From the melt

K20,A1,0~,6Si02 Na20yA1203,6Si02}

Na20,A1,0,,2Si02,~Na2S Na20,A1,0,,2Si0,,~NaC1

Na,0,AI,03,2S~0,,~Na,S0, Na2O,Al20,,2S~O2,~Na2COS

Na,0,A1,0,,2Si02 Cs20,A1,0,,2Si02 Li20,A1203,4Si02

2Zn0 ,Si02 ZrO,,SiO,

Complex boroalum~nosilicate 9Ca0,3P20,,.CaF2

Hydrothermal ; pyrolytic P yr 01 yt ic Hydrothermal; pyrolytic Hydrothermal; pyrolytic Hydrothermal High temp. and pressure Hydrothermal Hydrothermal Hydrothermal; pyrolytic Hydrothermal ; pyrolytic Hydrothermal; pyrolytic Hydro thermal

Ref. 46,47, a

a b C

d e e e

g 5 h 39 i 38

f

ULandise and Ballman, 135th Meeting Amer. Chem. SOC., Boston, April, 1959. Btockbarger, Discuss. Faraday Soc., 1949,5, 294. Won Nieuwenburg and Blumendahl, Rec. Trav. chim., 1931,50,989; Gruner, Amer. Min., 1936,21, 51 1. diMorey and Fenner, J. Amer. Chem. Soc., 1917, 39, 1173. 6Barrer and White, J., 1952, 561. fRobertson, Birch, and McDonald, Amer. J. Sci., 1957, 255, 115. gBarrer and McCallum, J., 1953, 4029. hIngerson, Morey, and Tuttle, Amer. J, Sci., 1948, 246, 31. iHayek, Bohler, Lechtleitner, and Petter, 2. unorg. Chem., 1958, 295, 241.

the crystals obtained are too small to be of commercial interest as gem materials, although once the conditions of formation are established, production of large crystals can usually be achieved if sufficient effort is devoted to the task. Of the materials listed, only three have been grown as

45 Nielsen, J. Appl. Phys., 1960, 31, 51s.

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WHITE : SYNTHETIC GEMSTONES 25

large crystals (above -1 cm. in length), namely, blende, fluorite, and zincit e .

BZende.-Zinc sulphide crystals have been grown from the vapour phase by two methods. First, by reaction of zinc vapour, carried by an inert gas, with hydrogen sulphide in an electric furnace,46 and, secondly, by distillation in a closed silica tube which is heated in a temperature- gradient regulated to achieve controlled nucleation and growth (Fig. 4B).47 Both methods are capable of giving long hexagonal prisms, although occasionally plate-like crystals are formed. Traces of activating agents may be incorporated in the crystals in order to modify the luminescent charac- teristics of the products. The crystals are of particular interest for the study of electroluminescence.

Fluorite.-Fluorite crystals have for many years been used in optical devices where ultraviolet transmission is important. The scarcity of large crystals of optical quality during the war stimulated attempts at the growth of calcium fluoride as well as of other halide crystals. The most successful crystals were first obtained by S t~ckba rge r ,~~ using a modifica- tion of the Bridgman technique for growth from the melt. As in the Bridgman technique, the crucible used is tapered to a point at the bottom in order to restrict nucleation to a single crystallite. The procedure consists in melting the fluoride in the upper part of a vertical tubular furnace which is sharply divided into two temperature zones. The hot upper zone is separated from the lower zone by an internal baffle which is large enough to permit lowering of the crucible slowly through the orifice into the cooler region until crystallation of the mass is complete. A vacuum-furnace with a graphite crucible is used and volatile impurities tend to be lost during melting; addition of small amounts of lead fluoride was found to provide a scavenging action and so improve the quality of the crystals.

Zinc&.-Zinc oxide has been studied with interest as a fluorescent material, as a semiconductor, and more recently as a piezoelectric material. Small crystals have been produced by vapour-phase growth but more recently large crystals have been grown hydr~thermally,~~ under conditions similar to those used for quartz.

Other Materials.-Numerous aluminosilicates have been synthesised hydrothermally in the extensive investigations carried out since 1900 by workers in several countries. Many of the species produced are of com- pounds which are classified as gem materials although they have little commercial significance from this viewpoint. In general, the materials produced are of simple composition, although it is of interest that perhaps the most complex gem material, tourmaline, has been synthesised.

The interesting sodalite-noselite family of felspathoids and the related 46 Piper, J. Chem. Phys., 1952, 20, 1343. 47 Reynolds and Czyzak, Phys. Rev., 1950,79, 543. 4a Stockbarger, Discuss. Faraday SOC., 1949,5, 294. 4 9 Laudise and Ballman, 135th Meeting Amer. Chem. SOC., Boston, April, 1959.

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26 QUARTERLY REVIEWS

cancrinite have been produced hydrothermally with crystal dimensions up to -5 mm. by spontaneous nucleation. The intercalated salts normally present can be replaced by varying the amounts of sodium hydroxide and water used, giving a series of crystals differing in their unit cell parameter^.^^ The related mineral hauynite (lapis lazuli) is manufactured commercially in the form of a fine powder which is used as a “blueing” agent for washing. Willemite is still an important cathodoluminescent material used in cathode-ray tubes, where the green fluorescence closely matches the optimum spectral response of the eye, while activated halogenophosphates based on the apatite composition are used on a large scale in tubular fluorescent lighting.

Properties of Gemstones

A primary concern of the gemmologist is the distinction between natural and synthetic materials. Although good-quality single crystals of a material such as coloured corundum are often difficult to distinguish from the natural counterparts, the label “synthetic” immediately limits the economic value of such materials. For this reason the term “cultured” has been applied to the ruby and emerald crystals produced by Chatham, implying that the method of production is basically the same as the natural process for the formation of these materials.

The distinction between natural and synthetic stones is usually simple for spinel and corundum crystals which are coloured to resemble gems of entirely different composition (e.g., “aquamarine,” “emerald,” “topaz,” “alexandrite”), since marked differences in the physical properties are involved. Distinction between the natural and the synthetic material of the same composition is more difficult, particularly if the colour effects are produced by the same trace elements. This subject has received con- siderable a t t e n t i ~ n , ~ ~ and various refined techniques are now available in gem-testing laboratories for this purpose. They are based on detailed optical examination and the measurement of physical properties.

Natural stones are seldom free from flaws and show a variety of effects associated with inhomogeneity, such as inclusions and colour striations, which are often used to characterise the natural stones with respect to locality of origin. Commonly the inclusions consist of crystals of other minerals, which if present in large numbers can often lead to the effects known as silk, asterism, or chatoyancy. Inclusions in synthetic crystals are usually different in character ; flame-fusion products often contain spherical or elongated gas bubbles and show curved colour striations perpendicular to the direction of Hydrothermal crystals, on the other hand, may contain two or three phase-inclusions which are similar

6o Barrer and White, J., 1952, 1561. 61 Anderson, Gemmologist, 1958, 27, 79; Webster, ibid., p. 170. 52 Herbert Smith, “Gemstones,” Methuen, London, 1958 ; Webster, J. Gemmology,

1957, 6, 101.

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WHITE : SYNTHETIC GEMSTONES 27

in character to those present in the natural stones. The study of inclusions, if used in conjunction with observations on colour, size, and crystal form, often leads to positive identification by the expert gemmologist.

Synthetic minerals are, in general, considerably purer than the natural species which often contain several major impurities as well as numerous trace impurities. These compositional differences can give rise to minor structural variations or discrepancies in physical properties ; for example, small differences in lattice parameters or in refractive indices can readily be determined by standard X-ray and optical techniques.

Analytical methods applied to gem materials must usually be restricted to non-destructive techniques, preferably involving only simple apparatus. For this reason, identification based on absorption spectroscopy is most useful. Colours in natural stones are usually produced by complex mixtures of impurities, and the absorption spectra thus often differ from those of synthetic stones, although the appearance of the stones may be similar.

Other methods of identification are based on differences in X-ray and ultraviolet fluorescent spectra and differences in the transmission to infrared or ultraviolet light detected by simple photographic means.

The most important physical properties of gem materials are colour, hardness and chemical stability, dispersion, refractive indices, special effects (colour zoning, asterism, etc.), and crystalline perfection. Of these, colour is perhaps the most important with respect to aesthetic appeal, although high refractivity and dispersion are desirable in colourless stones. Both synthetic rutile and strontium titanate have refractive indices of the same order as that of diamond, but have much greater dispersions (Table 8); consequently they are superior to diamonds as cut stones with regard to “fire”, but they are comparatively soft and are quickly abraded by the quartz particles which are a common constituent of dust. It is noteworthy that the majority of the minor gemstones are softer than quartz.

The scientific and industrial usage of gem materials is based usually on properties which are not necessarily connected with those listed above, although chemical stability and hardness are often additional assets.

The use of diamond, corundum, and garnet as abrasives, cutting materials, jewel bearings, and gramophone needles is well known, and the optical and fluorescent characteristics of other materials have been referred to above. Of considerable current importance is the use of gem materials for maser applications; for this purpose some of the relevant factors are the abiIity to incorporate a paramagnetic ion homogeneously in a stable lattice, the energy levels available for the ion, the structural symmetry of the host lattice, and the availability of large and comparatively perfect crystals.

Many of the gemstones listed here may be of potential interest for such applications, but the incomplete knowledge of the factors involved makes it difficult to predict theoretically the usefulness of a particular material.

Other solid-state investigations are concerned with the presence of im-

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28 QUARTERLY REVIEWS

perfections, vacant sites, or activating impurities for the stimulation of semiconductivity (e.g., diamond, rutile), electroluminescence (diamond, blende) and ultravi'olet luminescence (blende, apatite, willemite).

TABLE 8. Physical properties of gem materials. Material Crystal Hardness Refractive

system (Mohs scale) indices Diamond cub. Corundum hex. Spinel cub. Rutile tetr. Strontium titanate cub. Quartz hex. Beryl hex. Garnet cub. Yttrium iron garnet cub. Blende hex.

(cub.) Zincite hex. Fluorite cub. Feldspar (orthoclase) mon. Hauynite cub. Sodalite cub. Noseli te cub. Cancrinite hex. Jadeite mon. Pollucite cub. Spodumene mon. Willemite hex. Zircon tet. Apatite hex. Tourmaline hex.

10 9 8

6-6.5 d-6.5

7 7.5-8

6'5-7.5 -7 3.5"

4 4 . 5 4

6-6.5 5.5-6 5.5-6

5.5 5-6

6.5-7 6-5

6.5-7 5-55

7.5 4.5-5 7-7.5

*U = uniaxial. B = biaxial.

The use of synthetic quartz

2.4195 1.765, 1.773

1.717 2.616, 2.903

2.409 1.544, 1.553 1.575, 1.582 1-74-1 -79 >1.72

-2.5

2.013, 2.029 1.434

1.519, 1.523, 1.525 1.496 1-483

1 *48-1.495 1.496, 1.524

1.654, 1.659, 1-667 1.525

1.65, 1.66, 1.675 1.691, 1.719 1.923, 1.960 1.632, 1.648 -1.64

Dispersion Birefring- nB-nG ence* 0.044 - 0.018 0.008 U- 0,020 -

4 . 3 0.287 Uf -0.2 -

0.013 0.009 U+ 0.014 0.007 U-

0.024-4.057 - -

0.156 0.022 Uf

0.016 Uf 0.007 - 0.012 0.008 B-

- 0.029 U- 0.012 Bf

0.0 12 - 0.017 0.015 Bf

0.020 Uf 0.039 0.059 Uf

0.003 U- 0.017 0.018 U-

crystals as piezoelectric elements for oscillators, filter crystals, and frequency standaids is now well established. The availability of natural crystals for these purposes has limited the scale of commercial production, although pilot-scale plants have been operated, and two full-scale plants have been established in America.

Future Trends

The present interest in single crystals and growth techniques is likely to increase in the future, with the production of larger and more perfect crystals of the materials discussed in this Review as well as of other materials that have so far escaped attention. The application of different growth techniques to the present materials may be possible with the increasing availability of equipment that can withstand high pressures and temperatures. The growth processes tend to become closer to the natural conditions of formation, with a consequent closer similarity of the natural and the synthetic materials.

The publicity associated with the synthesis of diamond tends to mask the importance of the new technique involved. The possibility of making

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other gem-like materials by this method has already been demonstrated with the production of borazon. Similar high-pressure polymorphs, as well as new compounds, are to be expected, particularly i*n boride, carbide, nitride, and phosphide systems. Increasing interest in the physics and chemistry of high pressures is being shown in many countries, and the studies may well result in the production of new ranges of hard, refractory materials.

Although the newer synthetic gem materials (rutile and strontium titanate) are not yet popular in this country, they have received greater publicity abroad and are marketed under various trade names. At present, most available specimens are colourless, but coloured specimens have been prepared which are very attractive and should add to the interest of these materials. Their comparative softness is a disadvantage, but this may be overcome to some extent by the vacuum-deposition of a thin layer of alumina on the cut stones to provide resistance to abrasion.

Colouring of natural stones, particularly diamonds, by irradiation has been investigated and many interesting effects obtained. It does not seem likely, however, that this technique will be of commercial importance, particularly as the demand tends to colourless, rather than coloured, diamond.

It is interesting that one of the most costly of the gem materials, opal, has yet to be synthesised. Attempts to reproduce the effects in glass are recorded by P l i n ~ , ~ ~ and more recently plastic materials have been em- ployed. The problems presented in the synthesis of this material are probably as great as those for diamond.

53 Pliny, Book 37, ch. 6.

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