Prehnite

Prehnite is an acid orthosilicate of calcium and aluminium with the formula H₂Ca₂Al₂(SiO₄)₃. It is brittle with an uneven fracture and a vitreous to pearly lustre. Its hardness is 6-6.5, its specific gravity is 2.80-2.90 and its color varies from light green to grey or white. It is translucent.

Though not a zeolite, it is found associated with them and with datolite, calcite, etc. in veins and cavities of basic rocks, sometimes in granites, syenites or gneisses. It was first discovered in South Africa by Colonel Prehn, an early Dutch governor of the Cape of Good Hope colony.