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Alexandrites Directory, Info @ Tex.in
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Alexandrites Directory, Info
Content derived from Wikipedia article on Chrysoberyl
Chrysoberyl From Wikipedia, the free encyclopedia
Chrysoberyl
General Category Mineral Chemical formula Beryllium aluminium oxide, BeAl2O4 Identification Color Various shades of green and yellow; brownish, reddish; rarely, blue Crystal habit slender prisms and tabular form, dimensions are thin in one direction. Crystal system Orthorhombic 2/m2/m2/m Twinning Contact and penetration twins common, often repeated forming rosette structures Cleavage [110] Distinct, [010] Imperfect Fracture Conchoidal to uneven Mohs Scale hardness 8.5 Luster Vitreous Refractive index Biaxial (+) nα=1.745 nβ=1.748 nγ=1.754 Pleochroism Strong in alexandrite Streak White Specific gravity 3.5 - 3.84 Major varieties Alexandrite Color change; green to red Cymophane Chatoyant The mineral or gemstone chrysoberyl, not to be confused with beryl, is an aluminate of beryllium with the formula BeAl2O4. Chrysoberyl is transparent to translucent and sometimes chatoyant. An interesting feature of uncut crystals of chyrsoberyl are the cyclic twins called trillings. These twinned crystals have a hexagonal appearance, but are the result of a triplet of twins with each "twin" taking up 120 degrees of the cyclic trilling. The word chrysoberyl is derived from the Greek chrysos, "golden," and beryllos, of uncertain etymology.
Contents
1 Occurrence 2 Alexandrite variety 3 Cymophane variety 4 See also 5 References 6 External links
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Occurrence Chrysoberyl occurs in granitic rocks, pegmatites and mica schists; often it is found in alluvial deposits. It has also been found in contact metamorphic deposits of dolomitic marble with corundum, and in fluorine bearing skarns. Most chrysoberyl is recovered from river sands and gravels.
Alexandrite variety The alexandrite variety displays a color change (alexandrite effect) dependent upon light, along with strong pleochroism. Alexandrite results from small scale replacement of aluminium by chromium oxide, which is responsible for alexandrite's characteristic green to red color change. Alexandrite from the Ural Mountains in Russia is green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light. The optimum or "ideal" color change would be fine emerald green to fine purplish red, but this is exceedingly rare. Because of their rarity and the color change capability, "ideal" alexandrite gems are some of the most expensive in the world.
Alexandrite was first discovered in 1831 in an emerald mining region of the Ural Mountains in Russia. The name comes from Tsar Alexander II of Russia, on whose birthday the gem was discovered in that country. It was named "alexandrite" in his honor by the mineralogist Adolf Erik Nordenskiöld. It is an interesting coincidence that the Russian national colors are green and red, which may have originated from this gem.
The finest alexandrites were found in the Ural Mountains, the largest cut stones being in the 30 carat (6 g) range, though many fine examples have been discovered in Sri Lanka (up to 65 cts.), India (Andhra Pradesh), Brazil, Myanmar, and especially Zimbabwe (small stones usually under 1 carat (200 mg) but with intense color change). Overall, stones from any locale over 5 carats (1 g) would be considered extremely rare, especially gems with fine color change. Alexandrite is both hard and tough, making it very well suited to wear in jewelry.
The gem has given rise to the adjective "alexandritic", meaning any transparent gem or material which shows a noted change in color between natural and incandescent light. Some other gem varieties of which alexandritic specimens have been found include sapphire, garnet, and spinel.
Some gemstones described as lab-grown (synthetic) alexandrite are actually corundum laced with trace elements (e.g., vanadium) or color-change spinel and are not actually chrysoberyl. As a result, they would be more accurately described as simulated alexandrite rather than synthetic.
Synthetic alexandrite is used as an active laser medium. Alexandrite laser crystals tend to be round, with a pale brown tint.
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Cymophane variety
A chrysoberyl gemstone featuring a somewhat asymmetric, "native" oval mixed cut.Translucent yellowish chatoyant chyroberyl is called cymophane or cat's eye. Cymophane has its derivation also from the Greek words meaning wave and appearance, in reference to the chatoyancy sometimes exhibited. In this variety, microscopic tubelike cavities or needlelike inclusions of rutile occur in an orientation parallel to the c-axis producing a chatoyant effect visible as a single ray of light passing across the crystal. This effect is best seen in gemstones cut in cabochon form perpendicular to the c-axis. The color in yellow chrysoberyl is due to Fe3+ impurities.
Although other minerals such as tourmaline, scapolite, corundum, spinel and quartz can form "cat's eye" stones similar in appearance to cymophane, the jewelry industry designates these stones as "quartz cat's eyes", or "ruby cat's eyes" and only chrysoberyl can be referred to as "cat's eye" with no other designation.
See also List of minerals
References Klein, Cornelis; and Cornelius S. Hurlbut, Jr. (1985). Manual of Mineralogy, 20th ed., New York: Wiley. ISBN 0-471-80580-7. Weinstein, Michael (1958). The World of Jewel Stones. New York: Sheridan House.
External links Alexandrite guide Webmineral Mindat.org Retrieved from "http://en.wikipedia.org/wiki/Chrysoberyl" Categories: Beryllium minerals | Aluminium minerals | Oxide minerals | Gemstones
End of Wikipedia content, http://en.wikipedia.org/wiki/Alexandrite
Content derived from Wikipedia article on Synthetic Alexandrite
Synthetic alexandrite From Wikipedia, the free encyclopedia
Synthetic alexandrite has been created by the Floating Zone Method. This method is also used to synthetically grow a variety of crystals: YAG (Yttrium Aluminum Garnet) in white, blue, pink and several other shades, Ruby, Chrysoberyl and, most and foremost, Alexandrite. This method is similar to the pulling (Czochralski) method, when the crystal is pulled vertically, but, instead, here it is pulled horizontally, using a tungsten dish and slow-melting technique, where the dish is pulled over a period of time under a heating element.
The period of time depends on the size of the dish and the materials needed to be crystallized. In the case of alexandrite, the speed of the process is about 2 mm/hour. An average crystal being 18 cm long would take about 15 days to pull.
This method was developed by a Russian-Armenian scientist, Professor Bagdosarov, of the Institute of Crystallography, in the 1970’s and was widely used in production of white YAG for spacecraft and submarine illuminators, before finding its way into the jewelry scene. The actual method of Alexandrite’s production by this method was developed by Tairus’ Scientist, Vladimir Gurov, whom development process of other gems, such as Chrysoberyl and Ruby by this method are also attributed.
Flux grown alexandrite is more difficult to identify because the inclusions of undissolved flux can look like natural inclusions. Alexandrite grown by the flux-melt process will contain particles of flux, resembling liquid feathers with a refractive index and specific gravity that echo that of the natural material. Layers of dust-like particles parallel to the seed plate, and strong banding or growth lines may also be apparent. Some stones contain groups of parallel negative crystals. Flux grown alexandrites are more difficult to spot because the colors are convincing and because they are not clean. These stones are expensive to make and are grown in platinum crucibles. Crystals of platinum may still be evident in the cut stones.
Czochralski or pulled alexandrite is easier to identify because it is so clean. Curved striations visible with magnification are a dead give away. The color change in pulled stones has seen change from blue to red. Although the stones look nice, the colour change doesn´t resemble alexandrite from any deposit. Seiko synthetic alexandrites have a swirled internal structure characteristic of the floating zone method of synthesis. They have tadpole inclusions (with long tails) and spherical bubbles.
The Inamori synthetic alexandrite had a cat´s eye variety, which showed a distinct colour change. The eye was broad and of moderate intensity. Specimens were a dark greyish-green with slightly purple overtones under fluorescent lighting. The eye was slightly greenish-bluish-white and the stones were dull and oily. They appeared to be inclusion-free and under a strong incandescent light in the long direction, asterism could be seen with two rays weaker than the eye. This has not been reported in natural alexandrite. Under magnification, parallel striations could be seen along the length of the cabochon and the striations were undulating rather than straight, again not a feature of natural alexandrite.
The name Allexite has been used for synthetic Alexandrite manufactured by Diamonair Corp who maintain that their product is Czochralski-grown.
References Alexandrite.net contributors. Synthetic alexandrite production. In Alexandrite.net, Tsarstone collectors guide. September 07, 2006, 09:53 UTC. Available at: http://www.alexandrite.net/viewpage.html?id=ALXS-002-00011. Accessed November 04, 2006.
External links Alexandrite Synthetics and Imitations Retrieved from "http://en.wikipedia.org/wiki/Synthetic_alexandrite" Category: Synthetic minerals
End of Wikipedia content, http://en.wikipedia.org/wiki/Synthetic_alexandrite
Web Resources for Alexandrite
Alexandrite
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Different colors in one stone of Alexandrite
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