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Gemstones: Alexandrite



Alexandrite (Chrysoberyl) Gemstones


Alexandrite


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 Nikolaevich 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 and Malyshev emerald mines of Yekaterinburg (aka Ekaterinburg or Sverdlovsk) [2], which are situated along the eastern slopes of the Ural Mountains in Sverdlovsk Oblast, Russia.



Alexandrite was first identified as a variety of chrysoberyl by a Finnish mineralogist named Nils Gustaf Nordenskjöld in 1834, naming it "Diaphanite" for its unusual ability to "change" color [4]. Several years later, the stone was officially renamed "Alexandrite," either by Alexander Nikolaevich himself, or possibly by Lev Aleksevich von Perovski (1792—1856) who was a Russian nobleman, mineralogist and a Minister of Internal Affairs under Nicholas I.


Alexandrite & Czar Alexander

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" due to an optical phenomenon known as pseudochromatic coloration; caused by the way in which different color temperatures of ambient light are absorbed within the crystal. Due to the unusual way in which alexandrite absorbs certain elements of the color spectrum, it can look greenish-bluish-grey in daylight (cool, "emerald" color), and reddish-purple or 'raspberry red' under artificial light (warm, "ruby" color). On a molecular level, Alexandrite's characteristic green-to-red color change results from small-scale replacement of alumina by chromium oxide.



Alexandrite Rough from the Malyshev Emerald Mine, Russia

Alexandrite rough - Malyshev Emerald Mine, Russia

   Brazilian Alexandrite

Brazilian Alexandrite (photo: AfricaGems.com)


This rare variety of chrysoberyl also exhibits strong pleochroism, and being a member of the orthorhombic crystal system, Alexandrite (and chrysoberyl) can exhibit up to three unique colors (trichroism). The "trichroic" effect of orthorhombic, as well as monoclinic and triclinic minerals, is not directly related to Alexandrite's unique ability to appear as a totally different colored stone in differing spectrums of ambient light.


Alexandrite Orthorhombic Crystal Structure

Natural alexandrite is very rare, and the finest alexandrite crystals ever found have come from Russia's Tokovaya river deposit. The greatest Alexandrite specimen ever found is housed in Moscow's Fersman Mineralogical Museum.


Alexandrite (Chrysoberyl) Crystallography, Chemistry, Physical Properties

Crystal System orthorhombic
Crystal Habit tabular, striated, prismatic
Specific gravity (SG) 3.60 to 3.84
Mohs Hardness Scale 8.5
Toughness good
Fracture conchoidal to uneven
Cleavage distinct {011} perfect, indistinct {010}, poor {100}
Streak white
Chemical Composition BeAl2O4

Alexandrite (Chrysoberyl) Optical Properties

Optical Properties double refractive, sometimes chatoyant
Refractive Index 1.745
Birefringence 0.008 - 0.011
Pleochroism strong (X= red-purple, Y= orange, Z= green)
Surface Luster vitreous to greasy
Diaphaneity transparent, translucent, subtranslucent
Gem Color brown, green, greenish-yellow, reddish-purple, yellow

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

Purple Brazilian Alexandrite (photo: AfricaGems.com)

   Rough Alexandrite from Takovaya, Russia

Rough Alexandrite - Takovaya, Ekaterinburg, Russia


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.





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Bibliography and Reference on Alexandrite


2. History of Alexandrite www.alexandrite.net

3. Peter Bancroft, Russian Alexandrite www.palagems.com

4. Tsarstone Collector's Guide, Alexandrite or... Diaphanite? . www.alexandrite.net

5. Africa Gems, Brazillian Alexandrite . www.africagems.com

6. Judith Crowe, The Jeweler's Directory of Gemstones . DK Publishing.






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