Euclase

noun

1. A very brittle mineral of a pale-green color and high luster, crystallizing in prismatic crystals belonging to the monoclinic system.
2. A brittle gem occurring in light green, transparent crystals, affording a brilliant clinodiagonal cleavage. It is a silicate of alumina and glucina.
3. A monoclinic beryllium aluminium hydroxide silicate mineral, a product of the decomposition of beryl in pegmatites.