Gemstones: Alexandrite
Alexandrite (Chrysoberyl) Gemstones
Source: Brazil, Burma, India, Madagascar, Russia, Tanzania, and Zimbabwe
Birthstone: June (Alternate: Moonstone, Pearl)
The ultra-rare gemstone "Alexandrite" (aka Tsarstone or Czarstone) can trace its name back to the day of the stone's alleged discovery on April 29, 1834, the same day that a young Russian named Alexander Nicolajevitch II (soon to be Czar Alexander II - reigned 1855 to 1881) had his "coming of age" sixteenth birthday. Alexandrite was reportedly discovered along the banks of the Tokovaya River, at the Izumrudnye Kopi emerald mines of Yekaterinburg (aka Ekaterinburg or Sverdlovsk) [2], which is situated along the eastern slopes of the Ural Mountains in Sverdlovsk Oblast, Russia.
Alexandrite is a variety of the mineral chrysoberyl (a cyclosilicate), which is an aluminate of beryllium. Chrysoberyl is usually transparent to translucent, and is sometimes chatoyant (cat's eye effect). Alexandrite typically occurs within mica schists, (pegmatites) or within their secondary alluvial deposits. Alexandrite has a tabular, striated, and/or prismatic crystal habit, forming pseudo-hexagonal or cyclic ("iron cross" and "cog wheel") crystal twinning.
This mercurial gemstone has a unique ability to change color (pleochroism) due to pseudochromatic coloration which is caused by changes in the color temperature of ambient light. Because of alexandrite's unique ability to absorb certain elements of the color spectrum, it can look greenish-bluish-grey in daylight (cool), and reddish-purple or 'raspberry red' under artificial light (warm). Alexandrite's characteristic green-to-red color change results from small-scale replacement of alumina by chromium oxide.
Natural alexandrite is very rare, and the finest alexandrite crystals ever found have come from the Tokovaya river deposit. The greatest Alexandrite specimen ever found is housed in Moscow's Fersman Mineralogical Museum.
Alexandrite (Chrysoberyl) Crystallography, Chemistry, Physical Properties
Alexandrite (Chrysoberyl) Optical Properties
Brazilian & Indian Alexandrite
Brazilian Alexandrite occurs in pegmatites surrounding the mining towns of Novo Cruzeiro and Nova Era, in the Minas Gerais region of Brazil. The color of most Brazilian alexandrite does not shift to the green end of the spectrum as much as Russian alexandrite, although there have been recent discoveries at Nova Era that produce a more pronounced color-shift. These Nova Era stones tend to be small specimens that can be heavily occluded.
Other sources have been the Narsipatnam and Vishakhapatnam mines (Vishnakahaputnam), within the Vishakhapatnam District of the Andhra Pradesh state in central-eastern India. The Vishakhapatmam mines were closed after the 2004 tsunami.
Purple Brazilian Alexandrite (photo: © AfricaGems.com) |
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Brazilian Alexandrite (photo: © AfricaGems.com) |
There are also Alexandrite mines in the Deobhog region (Samunda) in the state of Chhattisgarh (Chattisgarh), north of Andhra Pradesh. Madagascar and Tanzania are also sources for Alexandrite. Due to its extreme rarity, quality specimens of Indian or Brazilian Alexandrite can sell for several thousand dollars per carat.
Synthetic Alexandrite
Natural Alexandrite is very rare. Most alexandrine found on the market today is synthetic. From the late 1800s, synthetic alexandrine was made using corundum which had been treated with vanadium to give it the characteristic color-change effect. Since the early 1970s, true synthetic alexandrite has been produced by using the "flux-melt", "floating point", "floating zone", and "pulled crystal" methods. There are characteristic "rain-like" inclusions in synthetic alexandrite that help with its identification. Most synthetic alexandrite is produced in Japan and Russia.
Bibliography and Reference on Alexandrite
2. History of Alexandrite  www.alexandrite.net
3. Peter Bancroft, Russian Alexandrite www.palagems.com
4. Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.
5. Africa Gems, Brazillian Alexandrite . www.africagems.com
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