What is Prehnite?
Prehnite is a soft-green to yellow-green calcium aluminium silicate, famous for botryoidal aggregates and elegant translucent cabochons.
Mineralogy and crystal structure
Reference formula: Ca2Al2Si3O10(OH)2. Prehnite has an orthorhombic framework and commonly forms curved aggregates rather than isolated perfect crystals. Colour and translucency vary with inclusions and alteration. Prehnite is classified with silicates and has a orthorhombic crystal system. Typical hardness is 6–6.5 on the Mohs scale and specific gravity is approximately 2.8–3.0.
Colour, lustre and habit
Typical colours include yellow-green, green, white, colourless, pale blue. The usual botryoidal, reniform, stalactitic habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Prehnite forms in or is associated with basalt cavities, low-temperature hydrothermal veins, gneisses and contact-metamorphic rocks. Important documented localities include Australia, South Africa, Mali, India, Canada, the United States and France.
Botryoidal green with a calm glow
Prehnite is commercially successful because it bridges specimen and jewellery markets: a cavity spray shows geology, while a polished cabochon shows its soft inner glow.
Collecting and care
Avoid sharp impact and acids; clean with lukewarm water and a soft brush, especially when the surface is botryoidal.
