A mineral of great hardness, and, when transparent, of much
   beauty. It occurs in hexagonal prisms, commonly of a green or bluish
   green color, but also yellow, pink, and white. It is a silicate of
   aluminium and glucinum (beryllium). The aquamarine is a transparent,
   sea-green variety used as a gem. The emerald is another variety highly
   prized in jewelry, and distinguished by its deep color, which is
   probably due to the presence of a little oxide of chromium.