beryl, mineral, a silicate of beryllium and aluminum, Be₃Al₂Si₆O₁₈, extremely hard, occurring in hexagonal crystals that may be of enormous size and are usually white, yellow, green, blue, or colorless. Beryl is commonly used as a gemstone. The refractive index is low, and the stones have little or no fire. The most valued variety of beryl is emerald. An aquamarine is a blue to sea-green beryl; morganites are rose-red beryls. It is the principal raw material for the element beryllium and its compounds.