Labradorite FeldsparThe Feldspar GroupFeldspar is a ubiquitous mineral that, usually in the form of small grains, makes up 50-60% of the content of the rocks of the Earth's crust. More precisely, it's agroupof related mineral species, which, in larger deposits of single crystal forms, are known as several familiar gemstones: amazonite, moonstone, sunstone, orthoclase and labradorite.The entire feldspar group is divided into two main branches, the potassium feldspars: microcline and orthoclase, and the sodium/calcium feldspars known as the plagioclase "series". A solid solution series, in mineralogical terms, is a set of mineral species which grade in composition, within the same basic chemical formula, through mixtures, from one pure end material to the other. In the case of the plagioclase feldspars the series runs from 100% albite (NaAlSi3O8) to 100% anorthite (CaAl2Si2O8) with labradorite in the near 50/50 range.LabradoriteLabradorite is translucent to opaque with light to dark grey body color, often with needle-like inclusions of black magnetite or ilmenite and usually showing some fracturing.This gem is the only species that can claim sole possession of an entire optical phenomenon, in this case "Labradorescence". Only Labradorite gems show this distinctive directionally-oriented surface display of one or more metallic looking spectral colors. The structural cause is the repeated thin layer (lamellar) twinning of its crystals which creates both diffraction and interference as light passes through and reflects from the parallel surfaces.One of the most singular aspects of this iridescence is its distinct directionality. Notice in the photo below, how certain faces show a silvery or blue "shiller" and the others do not. Any gem fashioned from this material must be carefully oriented so that this display shows to best advantage, and even then, it will be visible only at certain angles. The thickness and uniformity of the layers determines the color(s) to be seen.

[Labradorite rough]The name derives from the original mine site along the coast of Labrador, found at the beginning of the 19th Century and which is still productive, but India, Scandinavia, Madagascar and the US now provide additional supplies. The majority of specimens of this gem show a silvery blue to bright blue sheen. The three specimens below show the range from semi-transparency, through translucence to opacity seen in the species. Lapidary artists have long exploited the beauty of the material in cabochons and gem carvings. Faceted specimens, though rarely seen, have a distinctive and unusual beauty.

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INCLUDEPICTURE "http://www.bwsmigel.info/gem.essayJPGs/LS-449C.sm.jpg" \* MERGEFORMATINET [Labradorite gems]Truth in MarketingThere are no synthetics or simulants to worry about with this gem group and enhancements are rarely encounted. One related issue does bear mentioning however--> large quantities of a translucent white Labradorite which originates in India is widely sold under the misnomer "rainbow moonstone" at very modest prices. (True moonstone is a different, rarer and considerably more expensive, species of feldspar that has its own distinctive optical phenomenon.) As you can see, the material in question is no less attractive for bearing its improper name.

[White Indian Labradorite, aka "rainbow moonstone"]SpectroliteA particularly colorful deposit of Labradorite was discovered in Finland, and later mined elsewhere in Scandinavia, which shows not only blue, but green, gold and rarely red or violet sheen, and has been given its own variety name: "spectrolite", due to its resemblance to the color spectrum.

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INCLUDEPICTURE "http://www.bwsmigel.info/gem.essayJPGs/Spectrolite.pin.sm.jpg" \* MERGEFORMATINET [Spectrolite jewelry]Care and UseDue to its modest hardness (6 - 6.5), heat sensitivity, and cleavability this gem is relatively fragile and must be set, worn, and cleaned with care. That care will reward the owner many times over, however, as a high quality, well cut piece of labradorite or spectrolite is a joy to behold. Every movement creates a shifting pattern of surface colors, the brightest of which can rival those on the wings of tropical butterflies. Ultrasonic or steam cleaning is too risky and gems to be used in rings or bracelets should be given protective settings and worn infrequently. The best use for this gem is earrings, brooches and pendants which are worry free.ValueLabradorite is a gem bargain, as even the highest quality specimens are a fraction of the cost of comparably colored ammolites, precious opals or fire agates. The most valuable pieces of both labradorite and spectrolite are those with the brightest and most uniform color flashes, showing no "dead" areas. In premium gems the fracturing and inclusions are minimal and non-intrusive. Beyond that, the value of a piece lies in its size and in the artistry of the cutting or carving.Gemological PropertiesMakeup: An aluminum silicate: 30-50% Albite (NaAlSi3O8) and 70 - 50 % Anorthite (CaAl2Si2O8)Crystal system: TriclinicRefractive Index: 1.55 - 1.57Birefringence: .009Hardness: 6 - 6.5Toughness: PoorSpecific Gravity: 2.70 - 2.75Cleavage: Perfect in one direction, good in another (at right angles to each other)Fracture: Uneven to splinteryUV Reaction: Usually inertLuster: Vitreous

All text and images are Copyright of Barbara W. Smigel, 2012 unless otherwise indicated.No reproduction allowed without explicit permission.

Mexican OpalThere are so many different names in use for this type of opal: fire opal, jelly opal, crystal opal, cherry opal, girasol, etc. In my opinion, therefore, the best generalterm for this sort of transparent opal with body color ranging from colorless through yellow to orange to deep red and usually without play of color, is Mexican opal.Even thoughsome pieces originate from other locales such as the USA and Brazil. This terminology helps prevent confusion from the term "fire" which is often used to mean "play of color". Mexican opal thatdoeshave play of color is in its own small sub-category, and is properly referred to as "precious Mexican opal".All opals are hydrated silicates usually containing from 3 - 10% water; but stones from a few sources (notably Virgin Valley, Nevada) can be as high as 20% in water content. If play of color is present, its source is the same in Mexican opal as for the precious white and black opals. Namely, diffraction and interference of light rays which travel through the spaces between the tightly packed silica spheres of which it is made. Some pieces show their color play in reflected light (more common) and some, only in transmitted light. These latter stones, called "contra-luz" are quite rare and desirable.Facetors appreciate Mexican opal's ready availability in nice sized pieces, its reasonable price, and its wonderful range of highly saturated colors. Only the most transparent pieces lack the phenomenon of "opalescence", which is a slightly to moderately strong milky haze within the stone's interior, similar to that seen in rose quartz. Much has been said about this gem's tendency to craze, that is, to develop fine cracks due to dehydration, but the vast majority of the pieces on the market are stable. Reputable rough dealers screen out the unstable pieces, before sale, by subjecting them to prolonged high temperature and low humidity conditions.Naturally, as with all types of opal, stones are somewhat fragile, and not well suited for use in rings meant for hard everyday wear or in bracelets or cufflinks. Likewise, strong chemicals, ultrasonic vibrations, and abrupt dramatic temperature changes can cause damage. In earrings, pendants, tie pins, brooches and special occasion rings and with reasonable care, it does very well.Cleaning with a soft brush and warm soapy water is safe and effective. It is not advisable, in fact, it is likely to be harmful, to store opals in mineral oil or or glycerin and this practice willnotprevent crazing. Storage in water, although safe, has no protective benefit. It is wise, however, to store the stones and jewelry items in their own separate compartments to prevent scratching from harder gems and metals.

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INCLUDEPICTURE "http://www.bwsmigel.info/gem.essayJPGs/MO-3335B.2.jpg" \* MERGEFORMATINET [Bi-colored Mexican opal, contra-luz Mexican opal]ValueThe most desirable pieces are nearly completely transparent and, when colored, show a strong, highly saturated hue. Most reds in this variety are tinged with various degrees of orange, so pure, spectral red pieces are exceedingly rare and therefore higher priced. Play of color enhances the value of any stone dramatically. Cat'seyes and bi-colored stones also sell at a premium.Mexican opal commands its highest prices in Germany and in Japan where many of the Mexican stones are exported. Here in the US, it remains an excellent bargain, even in larger, custom cut pieces of finest color. Enhancements such as heating, filling or irradiation are at present unknown in this variety.Gemological DataMakeup: hydrated silicon dioxideLuster: vitreous to resinousHardness: 5.5 - 6Crystal structure: none, it is amorphousFracture: conchoidal to unevenCleavage: noneDensity: 2.15RI: 1.42 - 1.43Birefringence: none

All text and images are Copyright of Barbara W. Smigel, 2012 unless otherwise indicated.No reproduction allowed without explicit permission.

Chrysocolla ChalcedonyThe common mineral quartz, occurs both in the familiar single crystal varieties of amethyst, citrine, rose quartz, etc, and also in a number of aggregate forms. These aggregates, such as agate, jasper, and chalcedony, are made up of submicroscopic quartz crystals intermeshed together. When the quartz aggregates are translucent, and of a single color, they are known as chalcedonies. Examples of well known types of chalcedony are brown to orange carnelian, and apple green chrysoprase.Less common, and more valuable, is a type of chalcedony with vivid greenish blue color, frequently referred to as"gem silica"in the trade. More correctly it would be called chrysocolla chalcedony. Structurally it is composed of near colorless chalcedony that has been stained, on a microscopic level, by infiltration of solutions carrying the same copper salts which give color to the mineral chrysocolla.If it is evenly stained throughout, it has an intense, uniform, slightly to moderately greenish blue color. Chrysocolla itself, though beautifully colored, is far too soft and fragile (H = 2 - 4) to be useable for jewelry purposes. Additionally, pieces of pure chrysocolla generally have a chalky, crumbly texture, or occur as thin powdery crusts on the surface of a rock.

[Chrysocolla specimen: lovely but not recommended for jewelry use]The so-called, "gem silica", however, since it is actually a type of chalcedony with quartz's hardness of 7, and excellent toughness,isquite suitable for jewelry use. Sources include, Arizona, New Mexico, Mexico, Taiwan and the Philippines. Rarely, gem silica occurs in botryoidal (with a bubble-textured surface) or drusy (with a sugar-like crystal coating) form.

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INCLUDEPICTURE "http://www.bwsmigel.info/gem.essayJPGs/GSC1.jpg" \* MERGEFORMATINET [Chrysocolla chalcedony (aka "gem silica"): top quality cabochon, drusy cabochon in a pendant, botryoidal cabochon, carving]ValueThe most valuable specimens of this kind of material are those that are highly translucent, evenly colored, free from inclusions, and strongly saturated in color. People who may not realize the rarity of such stones, are, sometimes, taken aback by the relatively high price for what is afterall, a form of quartz, and a cabochon stone to boot. Highly translucent cabochons of the most vivid color may retail for as much as $100/ct.Increased demand and familiarity with this gem has been occasioned by top gem carvers and goldsmiths recently making this stone a "gem of choice". There has also been intense interest by Oriental collectors which has driven prices up as well. Those specimens which tend to greenish hues and which are opaque, included, or uneven in color are much less costly.Gemological Data:Makeup: microcrystalline or cryptocrystalline quartz, Si02, colored by copperLuster: vitreousHardness: 7Crystal structure: hexagonalFracture: conchoidal to granularCleavage: noneDensity: 2.60RI: 1.54Birefringence: 0.004, usually not detectablePleiochroism: none

All text and images are Copyright of Barbara W. Smigel, 2012 unless otherwise indicated.No reproduction allowed without explicit permission.

MorganiteMorganite is beryl colored by manganese impurities. Although violet and peach are possible colors, the most common and preferred color is pink. Heat and light will remove the yellow component from peach beryl so it is often heated to get "pinker" stones. It entered the American market in 1911 when Tiffany & Co. introduced it and named it in honor of J.P. Morgan. Original deposits from Madagascar are now worked out, but Brazil, Namibia and other locations produce rough. Growth tubes are a typical inclusion in beryl and often seen in Morganite. Very often near colorless specimens are offered as Morganite when they more properly should be labeled Goshenite (colorless beryl). Although it takes a larger stone to develop really good body color, smaller stones can be very brilliant. Like most beryls, Morganite makes an excellent jewelry stone requiring no special care.

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INCLUDEPICTURE "http://www.bwsmigel.info/gem.essayJPGs/M-393B.jpg" \* MERGEFORMATINET [Morganite gems]ValueMedium light to medium pink, clean stones with custom cuts are the most valuable. Very light and included stones are on the lower end of the value spectrum. As Morganite frequently occurs in larger crystals, there is not the exponential increase in price with size we see in so many gems. Paradoxically, smaller Morganites (if they show good color) can be more valuable than larger ones which often, in order show good color must be so large as to limit their reasonable use in jewelry. As is the case with aquamarine, there is a small but growing segment of collectors who prefer the unheated peachy color and are willing to pay a premium to get an untreated piece.Gemological DataMakeup: a beryllium, aluminum silcateLuster: VitreousHardness: 7.5Crystal structure: HexagonalFracture: conchoidalDensity: 2.80RI: 1.58 - 1.59Birefringence: .008

All text and images are Copyright of Barbara W. Smigel, 2012 unless otherwise indicated.No reproduction allowed without explicit permission.

CHAROITENamed for the only locale in which it is found, the Charo River Valley in the former Soviet Union, Charoite is one of the few gems that is so distinctive in its color and patterns that a gemologist can feel justified in making a "sight" identification. There's really no other material likely to be mistaken for it -- at least this is true until a synthetic or man-made simulant comes along some day.Like lapis lazuli, the gemstone that we call "Charoite" is actually a rock composed of several minerals including Charoite! Unlike lapis, though, it is usually nearly pure Charoite mineral, with only slight amounts of microcline feldspar, aergirine-augite and tinaksite. It is the mineral Charoite that gives this gem its unmistakable purple color which, often in the same piece, ranges from very light to medium dark purple and from translucent to opaque. The other distinctive aspect of its appearance is the swirling patterns that form due to its fibrous crystals being arrayed in complex interlocking patterns.First found, 325 miles North of the tip of Lake Baikal, in the 1940's and locally called "lilac stone", this gem was introduced to the Western gemstone marketplace as Charoite in the 1970's. It immediately made a large impact, both with traditional lapidaries and marketers who used it for decorative objects, carvings and cabochons, and, soon after, with metaphysical gem enthusiasts for whom it embodies a long list of healing and spiritual attributes.Charoite is formed from limestone by the process of contact metamorphism. Since this is a relatively common geologic phenomenon it is not completely clear why its distribution is so limited. Apparently the particular limestone in that area had unique chemical properties as did the intrusive rocks. So far, gemologists have not been able to ascertain the exact chemical or structural reason for its purple color. To say that the mineral Charoite is a silicate of complex compostion an understatement: one mineralogical source describes it as a hydrated potassium, sodium, calcium, barium, strontium, silicate hydroxyfluoride!As a gem it is reasonably tough with a hardness between 5 and 6 and no cleavage. Use in rings or bracelets is probably unwise, but most other jewelry uses are safe. It is somewhat heat sensitive, so steam cleaning should be avoided, as should ultrasonic processes. As with the majority of gems, the best cleaning tool is a soft brush, a mild detergent and warm water.One of the loveliest aspects of the best Charoite gems is a slight to moderate chatoyance which gives it a silky or pearly luster. This attribute, as well as the swirling patterns and distinctive purple color, is well demonstrated by the pieces below:

INCLUDEPICTURE "http://www.bwsmigel.info/gem.essayJPGs/CHA-4279B.jpg" \* MERGEFORMATINET [Charoite gems showing their characteristic color and chatoyance]VALUE FACTORS:Charoite is a gemstone bargain. Even the highest quality pieces are, at most, a few dollars a carat. Look for a lovely pattern, pleasing colors, a good polish and a shape that appeals to you, and you cannot go wrong. If the piece shows some chatoyance, that would add to its valueGEMOLOGICAL DATA:Makeup: A rock composed mainly of the complex silicate mineral CharoiteCrystal System: MonoclinicHardness: 5 - 6Density: 2.5 - 2.8Refractive Index: 1.55 - 1.56Birefringence: .009Fluorescence: LW, weak to inert; SW, weak to inertFracture: conchcoidal to splinteryLuster: vitreous to pearly or silky

All text and images are Copyright of Barbara W. Smigel, 2012 unless otherwise indicated.No reproduction allowed without explicit permission.

Tsavorite GarnetTsavorite, or transparent, green grossular garnet, was discovered in Kenya in the 1960's and given its trade name by Tiffany marketers based on the proximity of Tsavo National Park to the mine sites. It is one of the most sought after and valuable types of garnet.The geologic deposits in which it is found are difficult and expensive to mine and unpredictable in distribution so production as been sporadic. Political conflicts and trade issues further endanger the reliable supply. The crystals which are found show evidence of being affected by tremendous geologic forces, and as a result are seldom found large and clean. Cut specimens over 3 carats are exceedingly rare.The color ranges from lime green to emerald green to pure spectral green and is caused by high vanadium content. Rarely found better than eyeclean, typical inclusions are straight or angular corrosion growth tubes, fingerprints, feathers, veils and graphite inclusions. Like all types of garnet this stone is the birthstone for the month of January.

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INCLUDEPICTURE "http://www.bwsmigel.info/gem.essayJPGs/TG-2365J.jpg" \* MERGEFORMATINET [Tsavorite gems: brilliant cut trillion, cabochon, step cut emerald cut, marquis brilliants in a ring]ValueGiven the rarity, popularity, and sporadic supply of these stones, the overall trend in value since their introduction has been up, up, up. The best specimens are those that are pure spectral green in a medium dark to medium tone. Those that are darker or lighter than this are much less desirable. Price exponentially increases with size and a custom cut adds considerable value as the majority of specimens are native cut.

Gemological DataMakeup: a calcium aluminum silicate

Crystal system: CubicLuster: Vitreous

Cleavage: noneHardness: 7

Fracture: conchcoidal to uneven

Density: 3.61

RI: 1.74

Dispersion: .028

All text and images are Copyright of Barbara W. Smigel, 2012 unless otherwise indicated.No reproduction allowed without explicit permission.