A low temperature hydrothermal mineral of the zeolite group, this mineral is found in amygdalules and vanities in basalt. It is characterized chiefly by its cleavage, pearly luster on the cleavage face, and common sheaf-like groups of crystals. The name is derived from a Greek word meaning "luster," in allusion to the pearly luster.

Mineral Photos courtesy of R.Weller/Cochise College