Alexandrite is a rare variety of Chrysoberyl.
Alexandrite was discovered near Ekaterinburg in the Ural Mountains in Russia in 1830, and
was named after Alexander II. Other sources now include Sri Lanka, Burma, Brazil, Zimbabwe,
Madagascar and North America. The important feature of Alexandrite which makes it highly
unusual and collectable amongst gemstones is its strong colour change when viewed in different
lights. Many other stones show a similar property, but nowhere as strongly, and not with such
highly contrasting colours. It is hardly ever seen in the U.K. because of its rarity and high
price.
Color
Chrysoberyl is mainly green, yellow or brown. High quality alexandrite varies from grass green
to raspberry red, although some stones will tend towards olive green or brown, and others to
yellow. The red and green colouration is caused by the partial replacement of aluminium by
chromium. In daylight, stones appear green, whilst in tungsten lighting they will change color
to red. This effect is caused by a combination of factors. Alexandrite is bi-refringent
(doubly refractive), and bi-axial, so that it possesses three different refractive indexes in
its three different optical directions. Each of these three directions has a strongly different
absorption spectrum, causing different colours to be seen when viewed from different directions
in relation to the crystal structure, an effect known as pleochroism. Daylight contains high
proportions of blue light, tungsten lighting contains a higher balance of red light. The two
main optical directions of alexandrite absorb light of different wavelengths in considerably
different ways. If this sounds technical and complicated, it is because it is technical and
complicated. I will try to return to this paragraph later when I have worked out a simpler
sounding way to explain the phenomenon. Any suggestions welcome.
Simulants
There are many synthetic alexandrites in the market place. Many simulated alexandrites have
been produced, usually made of synthetic corundum, occasionally synthetic spinel. A Japanese
company also markets a synthetic alexandrite. Many of the simulants are quite large,
10 to 20mm , and are often set in 9ct gold dress rings. If real they would weigh about 5 to 10
carats each, and would be worth a great deal of money.
Genuine (natural) alexandrites are so rare that many jewelers live a lifetime and never see one.
Technical Characteristics
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Chemical Composition:
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BeAl2O4 - Beryllium Aluminium Oxide
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Hardness:
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8.5
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Specific Gravity:
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3.71 - 3.72
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Refractive Index (R.I.):
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1.745 - 1.757
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Bi-refringence:
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0.008 - 0.010
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Crystal Structure:
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Rhombic
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