GO 340 Gemstones & Gemology
ES 567 Gemstones of the World
Dr. Susan Ward Aber, Geologist & Gemologist
Emporia State University
Emporia, Kansas USA

http://academic.emporia.edu/abersusa/go340/alexandr.htm

Alexandrite

Alexandrite is one variety of the mineral chrysoberyl, named after Czar Alexander II of Russia.
It was discovered in 1831, in the Ural Mountains of Russia, and is composed of BeAl2O4 (Schumann, 1997, p. 98).

Alexandrite from the
Ural Mountains, image taken from
Amethyst
Mineral Galleries
Alexandrite is unusual because at its best, this gem is a grass green in daylight and raspberry red in artificial, incandescent lighting. The trace element chromium is responsible for both the red and green of alexandrite. This same trace element is responsible for the green of emerald and red of ruby when is substitutes for aluminum in the crystal structure. The coloring agents are dependent upon the wavelength of light and the crystal structure/chemical bonding; the chromium in alexandrite is such that the color changes with wavelengths of light, from natural sunlight or fluorescent lighting, where is appears green, to indoor incandescent light, where is appears red (Amethyst Mineral Galleries, 1995-6).
Alexandrite from the
Ural Mountains, image taken from
Amethyst
Mineral Galleries

Synthetic spinel
used to imitate
alexandrite.
(Mislabeled-blue
topaz) Image taken
from The Image
on synthetics.
Alexandrite is the birthstone for June, along with pearl. High quality, large alexandrite specimens are rare and expensive, and therefore this is a gem that has inspired imitations and synthetics. Alexandrite was successfully synthesized by 1973 and it is often imitated by synthetic corundum and synthetic spinel. The synthetic versions change from a gray/blue violet in daylight to a reddish-violet in incandescent lighting. The Chatham company was successful in synthesizing alexandrite in 1975.
Chrysoberyl can be yellow, green, brownish, as well as the red/green color change alexandrite. The hardness is 8.5 and specific gravity 3.70-3.78. It has good cleavage and falls within the orthorhombic crystal system. The transparency varies from transparent to opaque and it is doubly refractive, 1.746 and 1.763. Fluorescence is weak to none. Opaque chatoyant "milk and honey" chrysoberyl is known as cat's eye and this term used alone always refers to chrysoberyl. Any other chatoyant mineral must be prefaced with the mineral name (e.g., tourmaline cat's eye).
Brown chrysoberyl,
image taken from
Gem Hut.

Image taken from
Amethyst Mineral Galleries
The host rock for chrysoberyl is granite pegmatite, metamorphic schists, and secondary placer deposits. The Ural Mountain source is gone today, but other locales include Brazil, Sri Lanka, Zimbabwe, Myanmar, India, Madagascar, Tanzania, Tasmania, China, and in the US. This gem is very rare as the Russian mines were worked out by the 1900s (Genis, 1998). The only currently mined source of any quality alexandrite is from Brazil, where mining techniques are crude and 99% of the stones collected are rejected (Genis, 1998). Historically, the largest alexandrite was found in Sri Lanka, at 1,876 carats; while the largest cut stone is 66 carats, on display at the Smithsonian Museum. The flawless Hope chrysoberyl is a light green faceted stone of 45 carats on display in London.

To learn more about alexandrite visit websites created by former GO 340 students Kerrie Howard and Jacob Bray.

References

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This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: esu.abersusie@gmail.com Thanks for visiting! Webpage created: November 15, 2000; last update: August 30, 2012.

Copyright 1999-2012 Susan Ward Aber. All rights reserved.