a rare very hard greenish-yellow mineral consisting of beryllium aluminate in orthorhombic crystalline form and occurring in coarse granite: used as a gemstone in the form of cat's eye and alexandrite. Formula: BeAl2O4
any of a group of minerals composed principally of oxides of magnesium, aluminum, iron, manganese, chromium, etc., characterized by their hardness and octahedral crystals.
