Dioptase - CuSiO₃·H₂O

Habit: Emerald green or blue green. Commonly prismatic to rhombohedral crystals; also as indistinct crystalline aggregates or massive. Vitreous luster; transparent to translucent. Green streak.

Environment: Occurs as a secondary mineral in oxidized zones of copper deposits.

Etymology: From the Greek words for "through" and "to see," in allusion to the high visibility of its internal cleavage planes.