A mineral having composition BeAl₂O₄ and crystallising in the orthorhombic system. Chrysoberyl crystals are usually tabular parallel to the front pinacoid and frequently in pseudohexagonal twins (see illus.). There is good prismatic cleavage. The hardness is 8.5 (Mohs scale) and the specific gravity is 3.7–3.8. The lustre is vitreous and the colour various shades of green, yellow, and brown. There are two gem varieties of chrysoberyl. Alexandrite, one of the most prized of gemstones, is an emerald green but in transmitted or in artificial light is red. Cat's eye, or cymophane, is a green chatoyant variety with an opalescent luster. When cut en cabochon, it is crossed by a narrow beam of light. This property results from minute tabular cavities that are arranged in parallel position.