Chemical composition -- Beryllium aluminate.
Optics -- R.I 1.74-1.77, biaxial positive or negative.
Durability -- Hardness 8.5. Chrysoberyl is also exceptionally tough (resistant to breakage), so it produces some extremely durable gems.
Crystal structure -- Orthorhombic.
Specific Gravity -- 3.68-3.80.
Sources -- Very widespread, but some of the finest sources include Brazil, Siberia, India, Zimbabwe, Sri Lanka, and Myanmar (Burma).
Alexandrite is the most famous variety; it was found on the birthday of, and named for, Czar Alexander II of Russia. This transparent chrysoberyl varies in apparent color depending on the incident light -- selective absorption of different wavelengths of light cause it to appear green or blue-green in daylight or fluorescent light and red to purplish in incandescent light. Fine alexandrite is exceedingly rare and expensive, especially in sizes over a few carats. The finest color changes are seen in stones from Russia and Zimbabwe, but good stones are also found in Sri Lanka and Brazil.
Cat's eye chrysoberyl, often called simply catseye, is translucent yellow, brown, or green material containing very fine silk-like inclusions that produce a distinctive chatoyance. When cut en cabochon, this produces a very sharp linear reflection of light known as the cat's-eye effect. Fine, large catseyes are rare and costly. The ideal color for catseye is honey brown, and an interesting "milk and honey" effect is seen, in which light striking the stone at an angle causes a shadow effect -- the side of the stone opposite the light appears rich brown, and the side nearest the light appears yellowish white.
Ordinary transparent chrysoberyl is most often yellow, yellowish green, or brown in color. When well cut, it produces bright, durable gems that, while not often seen in jewelry, are quite attractive and relatively affordable.