Chrysoberyl, a mineral composed of beryllium, aluminum, and oxygen. It is a very hard crystalline substance and is commonly used as a gem. Some types have a yellow tint; others are various shades of green. Greenish varieties with a whitish pupil are called cat's-eye; opalescent ones are named cymophane. An emerald-green variety called alexandrite has the property of turning red when artificial light is passed through it. Alexandrite is a June birthstone.
Chrysoberyl is BeAl2O4. Hardness: 8.5. Specific gravity: 3.5 to 3.84.