Prehnite: Gemstone Information
Prehnite is Ca2Al2Si3O10(OH)2 and is most commonly found in translucent light to dark green or yellow reniform or globular masses of the orthorhombic system with a characteristic oily appearance. The tabular or prismatic, steeply pyramidal crystals are uncommon. The hardness is 6–6.5 and the SG 2.802–2.95. The RI for the alpha, beta and gamma rays is 1.611–1.632, 1.615–1.642, 1.632–1.665, biaxial positive with birefringence 0.021–0.033. Prehnite occurs as a secondary or hydrothermal mineral in veins or cavities in mafic volcanic rocks or in granite gneisses or syenites.The most attractive green specimens come from the Fairfax quarry,Centreville, Virgnia, USA; colourless crystals from the Jeffrey mine,Asbestos, Quebec, Canada.