Lithos 112S (2009) 619–624

Contents lists available at ScienceDirect

Lithos

j ourna l homepage: www.e lsev ie r.com/ locate / l i thos

Brown diamonds and high pressure high temperature treatment

David Fisher ⁎Diamond Trading Company, DTC Research Centre, Belmont Road, Maidenhead, SL6 6JW, UK

⁎ Tel.: +44 1628 771234; fax: +44 1628 770101.E-mail address: david.fisher@dtc.com.

0024-4937/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.lithos.2009.03.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 September 2008Accepted 2 March 2009Available online 19 March 2009

Keywords:DiamondHigh pressure high temperature treatmentPlastic deformationAbsorption spectroscopy

Much progress has been made in recent years towards greater understanding of the various point andextended defects that give rise to colour in diamond and how such defects can be modified via treatment tochange a diamond's colour. Such fundamental understanding has been vital in providing reliable means forgemmological laboratories to identify treated diamonds and ensure maintenance of consumer confidence indiamond. The application of high pressure high temperature treatment to remove a brown colour componenthas prompted significant research recently into the cause of brown colour. This article reviews progress todate and the evidence that indicates that clusters of around 60 vacancies are the defects responsible for theabsorption causing brown colour. The general principles and techniques applied to the detection of diamondssubjected to high pressure high temperature treatment are also discussed. Detailed comparison betweenphotoluminescence spectra from treated and untreated diamonds is vital in the identification of Type IIadiamonds, whilst absorption features provide a robust means of identification for Type I diamonds. Adetection scheme based on the shape of the platelet-related infrared absorption is presented.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

A significant proportion of gem quality natural diamonds has abrown component to their colour. Whilst a number of different defectcentres can give rise to brown diamonds, the main process associatedwith the generation of this colour is plastic deformation. Not allplastically deformed diamonds are brown, however, and, in spite ofmany years of speculation about and investigation into this problem,the specific defects associated with the brown colour have untilrecently remained a mystery.

The need to gain better understanding of this phenomenon hasbeen given added impetus by the discovery that high pressure hightemperature (HPHT) treatment of brown diamonds can remove thiscolour. In addition to the scientific understanding systematic studiesof such treatment can provide, there is also a strong commercialincentive to carry out such treatment on relatively low value brownstones to improve their colour and increase their value. Work in thisarea at the Diamond Trading Company (DTC) Research Centre formspart of the Consumer Confidence Technical Research (CCTR) pro-gramme. This initiative aims to provide reliable techniques for theidentification of such treatments that can be used in gemmologicallaboratories as a means of maintaining consumer confidence innatural diamonds.

This paper will illustrate the progress made to date in determiningthe defect producing brown, how this enables phenomena associated

l rights reserved.

with colour-change treatments to be explained and how this under-standing combined with more empirical measurements is used toestablish reliable detection criteria.

2. Origin of brown colour

2.1. Observations

Any model for the defect responsible for brown colour and themechanism by which it is generated must be consistent with thephenomena associatedwith it. The brown colour is usually found to beconcentrated in bands parallel to {111} planes and this is aconsequence of its association with plastic deformation. Diamond istypical of a face-centred cubic crystal in that the active slip system is{111} b110N, i.e. during plastic deformation, slip occurs on {111} planesin b110N directions. Brown colour can be found in both Type I(nitrogen containing) and Type II (nitrogen-free, or more accuratelylow nitrogen with a total nitrogen concentration of less than about1 at. ppm) diamonds. Additional absorption generated by otherdefects can lead to a modification of the observed colour, but this lackof dependence on diamond type suggests that nitrogen is not directlyinvolved in producing brown colour. Whilst plastic deformation isnecessary to produce brown coloration, not all plastically deformeddiamonds are brown. Dislocations can often be observed via theluminescence they, or the point defects decorating them, emit onexcitation with ultraviolet light (photoluminescence) or electrons(cathodoluminescence). Type IIa diamonds of all colours showsignificant dislocation densities, thought to have been generated byplastic deformation some time in their histories. It has been noted that

Fig. 1. Room temperature ultraviolet/visible/near infrared absorption spectra of brownType IIa diamonds from the range of DTC colour grades (1st colour palest brown, 6thcolour deepest brown).

620 D. Fisher / Lithos 112S (2009) 619–624

colourless Type IIa diamonds tend to show dislocations arranged inwhat look like polygonised networks (Martineau et al., 2004), whichare indicative of plastic deformation at some point in the diamond'shistory but followed by a period of annealing during which theoriginal dislocations glide and climb to arrange into a lower energynetwork configuration (Sumida and Lang, 1981).

In addition to the observed distributions of colour and dislocations,any proposed defect must generate electronic states in the diamondband gap capable of producing the observed absorption. A combina-tion of theoretical modelling of a defect's electronic properties andspectroscopic measurements allows this to be assessed. Brown colourin diamond is due to a continuously rising absorption extending fromthe near infrared region of the spectrum to the indirect band edge inthe ultraviolet at a wavelength of around 225 nm. Examples ofabsorption spectra from Type IIa brown diamonds of differentstrengths of colour (using the DTC rough diamond colour grades of1st colour (palest) to 6th colour) are shown in Fig. 1, where theconsistency of the shape of the absorption spectrum can be clearlyseen.

Fig. 2. Transmission electronmicroscopy images of (a) brown and (b) colourless Type IIa diammainly arranged in hexagonal networks consistent with low-angle grain boundaries in the

2.2. Dislocations

The observations on brown diamonds suggest that dislocationsgenerated during plastic deformation could be directly responsible forthe absorption states associated with this colour. Such an idea waspursued using transmission electron microscopy (TEM) imaging tostudy dislocation configurations in brown and colourless Type IIadiamonds, Type IIa diamonds being chosen to reduce or avoid effectsassociated with nitrogen and defects such as platelets. Significantdifferences were observed as illustrated in Fig. 2 (DTC ResearchCentre, unpublished work). Type IIa brown diamonds tend to showdislocation configurations typical of plastically deformed crystals,with dislocations generally lying along b110N or b112N directions andevidence of interaction between dislocations moving along different{111} slip planes (Fig. 2a). TEM imaging of colourless samples showsclear evidence of polygonisation of dislocations, with dislocationsbeing concentrated at low-angle grain boundaries surroundingregions with relatively few dislocations. The hexagonal arrangementof the dislocations in the boundaries is clearly visible in Fig. 2b and thisis consistent with a tilt-twist low-angle grain boundary in a face-centred cubic crystal (see for example Hull and Bacon, 1984). Theobserved difference suggested that specific dislocation configurationscould give rise to brown colour whilst those associated withpolygonisation would have no absorption associated with them. Thisidea was tested by imaging an originally brown diamond after HPHTtreatment to turn it colourless in the expectation that polygonisationwould occur, but this turned out not to be the casewith the dislocationarrangements remaining very similar to that seen in untreated brownsamples. Some rearrangement of the dislocations is observed and thiswould be consistent with a kinetic process whereby some movementtowards dislocation polygonisation is observed in the laboratory atrelatively high temperatures and short times and more advancedpolygonisation generated by lower temperature anneals for geologicaltime scales in the earth's mantle. This initial study was followed byextensive work by Willems of dislocations in initially brown samplesbefore and after HPHT treatment to remove the colour who found nosignificant change in either the dislocation configurations or theirconcentrations (Willems, 2006; Willems et al., 2006).

Any effect associated with the absorption could therefore only beon a smaller scale, i.e. associatedwith the bonding configuration at the

onds. Tangles of dislocations are seen in the brown diamondwhilst the dislocations arecolourless diamond.

621D. Fisher / Lithos 112S (2009) 619–624

dislocation core. Electron energy loss spectroscopy (EELS) was used totry to probe gap states specific to individual dislocations (Kolodzie,2003), but proved inconclusive as it was not possible to isolate effectsgenuinely associated with dislocations from those that might havebeen induced by the electron probe beam. Theoretical calculations ofthe electronic states associated with different dislocation corestructures showed that it is possible to produce absorption atdislocations (Fall et al., 2002). Dislocations lying in {111} planes indiamond are complicated by the fact that there are two unequalspacings between the planes and therefore two different dislocationcore structures depending on whether the extra half plane of atomsterminates at the closely spaced planes (a so-called glide dislocation)or the widely spaced planes (a shuffle dislocation). It was determinedthat only dislocations belonging to the shuffle set have electronicstates within the band gap that could give rise to absorption and thateven in these cases the gap states predicted were not entirelyconsistent with the wavelength dependence of the observed absorp-tion. A comparison of the absorption strength expected for thedislocation core sites of such shuffle dislocations with the dislocationdensities observed in TEM studies of brown diamonds (typicallyaround 2×109 lines/cm2). This comparison suggested that to obtainthe level of absorption observed all dislocation sites would have to beactive — a highly unlikely situation as glide dislocations areenergetically more favourable than shuffle dislocations.

2.3. Vacancy clusters

A clue as to the defect responsible for brown colour is provided byresults on HPHT treatment of Type Ia diamonds. HPHT treatment ofsuch diamonds at around 2000 °C results in the colour changing frombrown to yellow (Collins et al., 2000). Careful observation of thecolour distribution shows that the initial banded distribution of thebrown colour is matched by the final distribution of yellow colour. Inthis case the yellow results from absorption by the H3 centre, due to adefect consisting of two nitrogen atoms adjacent to a single vacancy(N–V–N centre). Initially the diamond contained significant concen-trations of A-centres (substitutional nitrogen pairs) so the N–V–Ncentre has been formed via the combination of a vacancy with an A-centre. The distribution of the H3 absorption therefore suggests thatsignificant numbers of vacancies have been generated in the brownareas on HPHT treatment. Dislocation climb provides a mechanism bywhich vacancies can be generated and at laboratory time scales thiswould usually require heating the crystal to temperatures above aboutone third the melting temperature (Hull and Bacon, 1984) and forsignificant climb up to three quarters of the melting temperature(Lang, 1973). Alternatively, the vacancies could be released as aconsequence of the breakdown of larger defects containing one ormore vacancies, similar, for example, to the release and capture ofvacancies at N–V centres in irradiation enhanced nitrogen aggregation(Collins, 1980). Type IIa diamonds are inherently very pure andtherefore contain only very low concentrations of detectable pointdefects. This gives few candidates for defects that could be sources ofvacancies. In brown Type IIa diamonds the only defect present insignificant concentrations appears to be that responsible for thecolour itself and this therefore raises the possibility that the defectcould be small voids in the crystal that can be considered to beaggregates of vacancies or vacancy clusters. Such vacancy clusterswould be present only in the brown regions and would break up athigh temperatures to release individual vacancies.

Such clusters are difficult to analyse experimentally, being toosmall to be routinely imaged in TEM. Attempts have been made todetect them using defocused imaging techniques and comparisonwith image simulations based on predicted structures for the vacancyclusters (Bangert and Barnes, 2007). Such measurements are at thelimit of the capability of the technique and are difficult to quantify, butinitial results appear consistent with the presence of vacancy clusters

only in diamondswith brown colour at concentrations consistent withthose determined using other techniques. Further work applying thistechnique to a wider range of samples should help to confirm thesefindings.

The technique most suited to detection of vacancy clusters ispositron annihilation (Saarinen et al., 1998). Positrons are notnormally stable within crystals due to the high electron density. Ifpositrons are introduced into a crystal they therefore have a very shortlifetime, annihilating with electrons to produce a pair of 511 keVgamma rays. Detection of these gamma rays allows positron lifetimesto be measured. For relatively defect free diamond this lifetime is ofthe order of 100–110 ps. If open volumes are present within the crystalthese can result in an increase in the measured positron lifetimes,with effects being detectable from open volumes of one vacancyupwards. In diamond single vacancies have been found to increase thepositron lifetime to 140–150 ps (Pu et al., 2000) and lifetimes in excessof this are associated with larger open volumes, the size of which canbe estimated from the measured lifetime. Measurements on naturalbrown Type IIa diamonds have identified a dominant lifetimecomponent of around 400 ps. Assuming an approximately sphericalvoid this lifetime is consistent with a defect consisting of around 60vacant lattice sites. This long lifetime was not observed in colourlessdiamonds of the same type and HPHT treatment of a brown diamondwas found to remove this long lifetime component (Avalos andDannefaer, 2003). There is therefore experimental support for linkingbrown colour in diamond to the presence of vacancy clusters. There isstill a need to show a correlation between the annealing behaviour ofthe absorption responsible for the brown colour and the long lifetimecomponent in the positron data and we are currently working on anexperiment aimed at providing this correlation. The analysis carriedout to date suggests strongly that brown colour in diamonds isassociated with absorption from vacancy clusters.

There is also theoretical support for a link between vacancyclusters and the absorption responsible for brown colour. The voidswould be expected to have surfaces that would reconstruct to formlow energy {111} and {110} surfaces. The structure of the {111} surfaceand the electronic states associated with it have been modelledtheoretically by the construction of a vacancy disk in the {111} planevia the removal of a pair of atomic layers (Hounsome et al., 2006).Formation energy calculations predict the reconstruction of the {111}surfaces to form π-bonded chains and these give rise to electronicstates in the band gap. Calculation of the associated wavelengthdependent absorption associated with these gap states gives reason-able agreement with the observed absorption, further supporting theidea that vacancy clusters are responsible for brown colour. Thecalculations also suggest that if vacancy disks themselves werepresent in the diamond then they would be sufficiently stable tosurvive up to quite high annealing temperatures as is observed for theannealing behaviour of the brown colour. These disks would not havethe long lifetime component observed in positron annihilation, butwould have one more consistent with their shortest dimension (i.e.around 1–2 lattice spacings). Such short lifetimes have been observedin brown diamonds, but their annealing behaviour has not been foundto correlate with the strength of the brown colour (Avalos andDannefaer, 2003). Vacancy clusters remain, therefore, the most likelydefect to account for brown colour in natural diamonds.

2.4. Generation of brown colour in natural diamonds

Having identified a defect consistent with the annealing behaviourand absorption properties of brown colour, there remains the need tolink the production of vacancy clusters with plastic deformation. Inthe early stages of plastic deformation low concentrations ofdislocations are generated and there is little interaction betweenthem, but such interaction becomes very important as the dislocationconcentration increases. Of particular importance is the interaction

Fig. 3. Absorption spectra of Type II diamonds showing the main changes induced byHPHT treatment. The initially brown Type IIa diamond is turned pink at intermediatetemperatures, via the production of the broad absorption bands centred at 390 and550 nm, and colourless at high temperatures. Type IIb diamonds are turned blue onHPHT treatment as the absorption associated with brown colour is removed and thatdue to boron increases, as seen by the increased absorption to long wavelength. Spectraare offset for clarity.

622 D. Fisher / Lithos 112S (2009) 619–624

between dislocations on different slip planes as this can produce localsegments of a dislocation that are not capable of gliding under theaction of an applied shear stress. These segments can only move viaclimb or non-conservative motion and during this the dislocationsegment must generate vacancies (or interstitial carbon atoms). Sucha process requires higher stress than that associated with glide but hasthe potential to generate trails of high concentrations of vacancies. Thevacancies can then aggregate with each other to form clusters ofgradually increasing size (a process known as Ostwald ripening(Lifshitz and Slyozov, 1961; Wagner, 1961)), the final size of thecluster being dependent on the deformation conditions and thestability of the cluster so formed. Such a process has been observedexperimentally during plastic deformation in silicon (Leipner et al.,2006), but is difficult to replicate in the laboratory in the case ofdiamond. Density functional modelling of such vacancy clusters indiamond (Hounsome et al., 2005) shows that the energy per vacancyreduces as the size of the cluster increases and that clusters are morestable than chains of vacancies. Thus we have a mechanism for theformation of vacancy clusters that would explain their presence inplastically deformed crystals.

The geological conditions that have brought about the generationof brown colour must involve an environment likely to exert sheerstresses on the diamond. The most likely stage this would occur isduring emplacement, where there is impact with surrounding rocksand gradual solidification of the kimberlite. This late stage plasticdeformation is supported by the observation that some of the defectsfound to decorate dislocations are relatively unstable thermally andwould not be expected to survive significant periods of time at mantleconditions (Collins et al., 2000). Brown diamonds are therefore likelyto be those that have experienced plastic deformation late in theirhistories. The mantle itself is also not a benign environment in termsof its potential to plastically deform diamonds. Diamonds couldtherefore be plastically deformed quite early in their history.Subsequent annealing of these stones could then remove the vacancyclusters, and therefore the brown colour, and could also result in therearrangement of dislocations to form polygonised networks. It ispossible that such networks and the associated misoriented grainscould make the diamond more resistant to subsequent deformationand this would account for the high occurrence of colourless Type IIadiamonds containing dislocation networks. Brown colour could begenerated in such stones if during emplacement they experiencedparticularly high levels of stress, but the generation of brown colourwould be more likely in stones without the dislocation networks. TEMinvestigation of a wider range of colourless and brown stones shouldhelp in determining whether the proposed mechanisms areconsistent.

3. HPHT treatment

It is possible to remove the brown component of a diamond'scolour by annealing at high temperatures under a stabilising pressureto prevent graphitisation. Commercial exploitation of this techniquefirst emerged in 1999 when General Electric in partnership with

Table 1Summary of the main colour changes induced by HPHT treatment associated withdifferent diamond types and treatment conditions.

Diamond type

Type IIa Type IIb Type IaAB Type IaB

Starting colour Brown Brown/grey

Brown Brown

Colour after HPHTtreatment

Intermediatetemperature

Pale brownor pink

Yellow/green

Near-colourless

Hightemperature

Colourless Blue Yellow/orange

Yellow

Lazare Kaplan International launched their own range of HPHT treateddiamonds, now branded as Bellataire Diamonds. A number ofcompanies now offer HPHT treated stones or a service to carry outthe treatment on clients' stones.

3.1. HPHT treatment — colour changes

Whilst HPHT treatment can be applied to any diamond, work inthis area has concentrated on the more commercially importantcolour changes involving the removal of a brown component.Resulting colour effects are strongly dependent both on type (i.e. onnitrogen content) and treatment conditions as summarised in Table 1.Absorption spectra for Type II and Type Ia diamonds are shown in Figs.3 and 4 respectively. In Type IIa diamonds, HPHT treatment atsufficiently high temperatures will completely remove the brownresulting in a colourless stone in the majority of cases. A pale yellowstone can be produced under such circumstances as the hightemperatures involved are sufficient to produce dissociation of anylow concentrations of aggregated nitrogen in the diamond generatinga low level of single substitutional nitrogen (Fisher and Spits, 2000).HPHT treatment at intermediate temperatures will usually result in areduction in the brown colour, but, in a limited number of cases, this isalso accompanied by the production of two broad absorption bandscentred at around 390 and 550 nm. These bands result in a pink colourand are the same absorption bands as observed in natural pink

Fig. 4. Absorption spectra of Type IaAB diamonds showing the main changes associatedwith HPHT treatment. The main nitrogen-containing centres associated with thegeneration of yellow colours in the treated stones are labelled.

Fig. 5. Plot of the platelet peak full width at half maximum (FWHM) as a function ofcentre position for untreated and HPHT treated yellow/green Type IaAB diamondswhere the dominant absorption feature is the H3 centre.

623D. Fisher / Lithos 112S (2009) 619–624

coloured diamonds. A small proportion of nitrogen-free diamondscontains detectable quantities of boron, so-called Type IIb diamonds.Where the absorption responsible for brown colour is also presentthese stones usually show a grey/brown colour due to the combina-tion of brown colour and blue colour from the boron-relatedabsorption. Removal of the brown component by HPHT treatmentresults in a blue stone, the blue colour usually being enhanced by thefact that there is an increase in the concentration of neutral boron.This latter effect is thought to be due to the removal of a defect that iscompensating the boron and placing it in an alternative charge state.Data suggest that the vacancy clusters may well be the compensatingdefect and that these act as donors converting the boron into itsnegative charge state.

Colour changes in Type I diamonds are dominated by nitrogen-containing defects, with yellow colours being produced from brownstarting material (Type IaAB in Table 1). Example absorption spectrafrom such stones before and after HPHT treatment are given in Fig. 4.At intermediate temperatures a yellow to yellow-green colour isproduced due to H3 absorption as discussed in Section 2.3. This centrealso luminesces strongly in visible light, giving the stones a fluorescentgreen appearance. The H3 centre is relatively unstable and so is notproduced strongly in stones which are HPHT treated at highertemperatures. Here the dominant absorption centres are N3 (from adefect consisting of three nitrogen atoms surrounding a commonvacancy) and strong continuous absorption to the short wavelengthregion of the visible spectrum due to single substitutional nitrogen.The combination of these two absorption centres results in strongyellow to orange colours. Type IaB stones represent an exception tothis general trend for nitrogen-containing diamonds, where it hasbeen possible to produce near-colourless stones via HPHT treatment(Deljanin and Fritsch, 2001). This is due to the absence of A-centres,which could lead to the formation of H3 centres and yellow colour onHPHT treatment at intermediate temperatures, and the enhancedthermal stability of the B-centre over the A-centre, which prevents thegeneration of single nitrogen centres via dissociation providedtreatment temperatures are kept relatively low.

3.2. HPHT treatment — detection

Detection of HPHT treatment relies upon a careful comparison ofthe characteristics of such stones with those of untreated naturaldiamonds of similar colour and type. Research at the DTC has providedvital input to the establishment of reliable detection techniques viaboth the analysis of known natural benchmark samples from theextensive DTC intake and systematic experiments exploring the rangeof possible treatment conditions and starting material.

Detection in the case of Type I stones is generally carried out usingabsorption spectroscopy. The combination of nitrogen-related absorp-tion centres present in HPHT treated stones is unlike that usuallyencountered in untreated stones. StrongH3 absorption from the N–V–Ncentre is very rare in natural diamond. HPHT treatment tends also toproduce N–V–N centres in their negative charge state giving H2 as wellas H3 absorption. This combination is extremely rare naturally andwould be considered very suspicious. N3 and single nitrogen centres arethe dominant cause of colour for higher temperature treatments.Aggregation of nitrogen in natural diamond tends to follow aprogression whereby single substitutional nitrogen first aggregates toform pairs (A-centres) then, once this transformation is complete, thepairs aggregate further to form B-centres (groups of our nitrogen atomssurrounding a common vacancy). N3 centres are generated during thisaggregation of A-centres to B-centres. It is therefore extremely unusualto find an untreated stone that exhibits absorption features from bothsingle nitrogen and either N3 or B-centres. The only exception to this issome zoned samples where an older aggregated Type IaAB core hasbeen overgrown and this final growth, being much younger, is either

Type Ib or Type Ib/IaA. The clear zoning in such samples makesseparation from HPHT treated stones possible.

In the few cases where the above characteristics prove incon-clusive, it is possible in addition to use other characteristics notdirectly influencing the colour, thereby arriving at a reliable decision.One useful characteristic is the platelet-related infrared absorptionpeak. Platelets are layers of interstitial carbon atoms generated as a bi-product of nitrogen aggregation (Woods, 1986). The peak is generallyasymmetric and its position has been found to be related to the size ofthe platelets (Clackson et al., 1990). Fig. 5 shows the results of acomparison of the fit parameters for platelet peaks from untreatedand HPHT treated Type I yellow-green samples. It can be seen that fora given peak centre position, the width of the platelet peak is broaderfor the HPHT treated samples, indicative of a wider distribution ofplatelet sizes. Such analysis can provide a useful indication as towhether a stone has been treated, although some care must beexercised as cases of untreated stones have been found that lie in thetreated region. Generally a number of techniquesmust be combined toprovide reliable identification of a stone.

In the case of Type II stones, detection is complicated by the factthat such stones are inherently low in impurities and thereforerequires more sensitive techniques. Laser excited photoluminescencespectroscopy is a vital tool in the identification of such treatment andis used to analyse impurities at the parts per billion level in order toassess subtle differences that exist between the impurity contents ofuntreated and HPHT treated Type II stones. Such analysis was, untilrecently, beyond the scope of most gemmological laboratories, but inresponse to this challenge a number of the major laboratories nowhave the capability to apply this technique to the reliable identifica-tion of HPHT treated stones. Thus a number of criteria are used incombination to arrive at a decision on a particular stone. For example,in some cases, it is not simply the presence or absence of a feature thatdetermines whether the stone has been treated, but the charge stateof the defect, as indicated in work on the nitrogen-vacancy centre(Fisher and Spits, 2000).

4. Conclusions

Whilst this work is still in progress, current thinking regarding thedefect responsible for brown colour in plastically deformed naturaldiamonds increasingly indicates that vacancy clusters are responsible,with good agreement having been obtained between experimentalobservations and theoretical predictions. This work also highlights theimportant role that a good understanding of the fundamentalproperties of diamond plays in assisting in the development of

624 D. Fisher / Lithos 112S (2009) 619–624

reliable identification criteria for treated stones. Whilst much can beachieved using a purely empirical approach, the fundamental under-standing allows the potential of a particular treatment method to beaccurately assessed and detection criteria to be drawn up that arerobust to future developments. Maintenance of consumer confidencein the area of HPHT treatment has been extremely important as thecolours generated by such treatment are those that command thehighest prices in untreated stones. The reliable detection of HPHTtreated Type IIa diamonds is of particular importance. Whilst theoverall proportion of Type IIa stones is generally around 1–2%(Harlow, 1998), their proportion increases with diamond size. Tablesof the world's largest polished diamonds include a much higherproportion of Type IIa stones, including some of the largest and mostfamous diamonds: Golden Jubilee — 545.7 carats, Cullinan I — 530.2carats, Cullinan II — 317.4 carats, Centenary — 273.8 carats, De BeersMillennium Star — 203.0 carats (Balfour, 1997; Scarratt and Shor,2006; King and Shigley, 2003). This research and the robust andreliable means of identification generated help safeguard the reputa-tion of some of the world's most valuable diamonds.

Acknowledgements

Members of the CCTR team at the DTC are thanked for theircontributions to this work. Much of the fundamental research wascarried out in collaboration with a number of DTC sponsored/supported University groups, whose contributions are gratefullyappreciated. These include: Bob Jones, Luke Hounsome, Naomi Fujita,Alex Blumenau (Exeter University); Jussi-Matti Mäki, Filip Tuomisto,Kimmo Saarinen (Helsinki University of Technology); Bert Willems,Gustaaf Van Tendeloo (University of Antwerp); Rachel Barnes, UschiBangert (Manchester University).

References

Avalos, V., Dannefaer, S., 2003. Vacancy-type defects in brown diamonds investigated bypositron annihilation. Physica B 340–342, 76–79.

Balfour, I., 1997. Famous Diamonds, 3rd edition. Christie, Manson and Woods Ltd.,London.

Bangert, U., Barnes, R., 2007. Electron energy loss spectroscopy of defects in diamond.Physica Status Solidi (a) 204, 2201–2210.

Clackson, S.G., Moore, M., Walmsley, J.C., Woods, G.S., 1990. The relationship betweenplatelet size and the frequency of the B' infrared absorption peak in Type Iadiamond. Philosophical Magazine B 62, 115–128.

Collins, A.T., 1980. Vacancy enhanced aggregation of nitrogen in diamond. Journal ofPhysics C: Solid State Physics 13, 2641–2650.

Collins, A.T., Kanda, H., Kitawaki, H., 2000. Colour changes produced in natural browndiamonds by high-pressure, high-temperature treatment. Diamond and RelatedMaterials 9, 113–122.

Fall, C.J., Blumenau, A.T., Jones, R., Briddon, P.R., Frauenheim, T., Gutierrez-Sosa, A.,Bangert, U., Mora, A.E., Steeds, J.W., Butler, J.E., 2002. Dislocations in diamond:electron energy loss spectroscopy. Physical Review B 65, 205206.

Deljanin, B., Fritsch, E., 2001. Another diamond type is susceptible to HPHT: rare type IaBdiamonds are targeted. Professional Jeweler, October 2001, pp. 26–29.

Fisher, D., Spits, R.A., 2000. Spectroscopic evidence of GE-POL HPHT-treated naturaltype IIa diamonds. Gems & Gemology 36, 42–49.

Harlow, G.E., 1998. What is diamond? In: Harlow, G.E. (Ed.), The Nature of Diamond.Cambridge University Press, Cambridge, pp. 5–22.

Hounsome, L.S., Jones, R., Martineau, P.M., Shaw, M.J., Briddon, P.R., Oberg, S., Blumenau,A.T., Fujita, N., 2005. Optical properties of vacancy related defects in diamond.Physica Status Solidi (a) 202, 2182–2187.

Hounsome, L.S., Jones, R., Martineau, P.M., Fisher, D., Shaw, M.J., Briddon, P.R., Oberg, S.,2006. Origin of brown coloration in diamond. Physical Review B 73, 125203.

Hull, D., Bacon, D.J., 1984. Introduction to Dislocations, 3rd edition. Pergamon Press,Oxford.

King, J.M., Shigley, J.E., 2003. An important exhibition of seven rare gem diamonds.Gems & Gemology, 39, 136–143.

Kolodzie, A.T., 2003. EELS at dislocations in diamond. Unpublished PhD thesis,University of Cambridge.

Lang, A.R., 1973. The properties and observation of dislocations. In: Hartman, P. (Ed.),Crystal Growth: An Introduction. North Holland, Amsterdam, pp. 444–515.

Leipner, H.S., Mikhnovich Jr., V.V., Bondarenko, V., Wang, Z., Gu, H., Krause-Rehberg, R.,Demenet, J.-L., Rabier, J., 2006. Positron annihilation of defects in silicon deformedat different temperatures. Physica B: Condensed Matter 340–342, 617–621.

Lifshitz, I.M., Slyozov, V.V., 1961. The kinetics of precipitation from supersaturated solidsolutions. Journal of Physics and Chemistry of Solids 19, 35–50.

Martineau, P.M., Lawson, S.C., Taylor, A.J., Quinn, S.J., Evans, D.J.F., Crowder, M.J., 2004.Identification of synthetic diamond grown using chemical vapour deposition(CVD). Gems & Gemology, 40, 2–25.

Pu, A., Bretagnon, T., Kerr, D., Dannefaer, S., 2000. Positron annihilation investigation ofvacancies in as-grown and electron irradiated diamonds. Diamond and RelatedMaterials 9, 1450–1463.

Saarinen, K., Hautojärvi, P., Corbel, C.,1998. Positron annihilation spectroscopy of defects insemiconductors. In: Stavola, M. (Ed.), Identification of Defects in Semiconductors,Semiconductors and Semimetals, 51A. Academic, New York, pp. 209–285.

Scarratt, K., Shor, R., 2006. The Cullinan diamond centennial: a history and gemologicalanalysis of Cullinans I and II. Gems & Gemology 42, 120–132.

Sumida, N., Lang, A.R., 1981. Cathodoluminescence evidence of dislocation interactionsin diamond. Philosophical Magazine A 43, 1277–1287.

Wagner, C., 1961. Theorie der Alterung von Niederschlagen durch Umlosen (Ostwald-Reifung). Zeitschrift für Elektrochemie 65, 581–591.

Willems, B., 2006. Structural defects and colour-treated diamond: a transmissionelectron microscopy study. Unpublished PhD thesis, University of Antwerp.

Willems, B., Van Tendeloo, G., Martineau, P.M., Fisher, D., Van Royen, J., 2006. Dislocationdistributions in brown diamond. Physica Status Solidi (a) 203, 3076–3080.

Woods, G., 1986. Platelets and the infrared absorption of type Ia diamonds. Proceedingsof the Royal Society London A 407, 219–238.