- (n.) 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.