葡萄石

Prehnite is a phyllosilicate that almost always grows as rounded, grape-like (botryoidal) aggregates and stalactites rather than single crystals. Its soft, glowing apple-green, yellow-green and yellow colours, and its slightly translucent, almost jade-like appearance, have made it a long-standing favourite for both specimens and cabochons.

Identifying prehnite

Prehnite is fairly hard (6–6.5) and forms smooth, reniform (kidney-shaped) botryoidal surfaces with a waxy to vitreous lustre. A classic and beautiful association is prehnite hosting slender black or dark-green epidote crystals that pierce through the green mounds. It forms in cavities in basalt and other volcanic rocks and in some metamorphic settings.

Where it is found

Mali (the Kayes region) and Australia are renowned for gemmy green prehnite, and Scotland is the historic source where the mineral was first described. Prehnite also occurs in the volcanic and metamorphic rocks of several regions of China and elsewhere worldwide.

What collectors look for

Glowing, translucent green botryoidal pieces — especially the dramatic combinations of green prehnite with radiating black epidote — are the most prized. Colour, translucency and a smooth undamaged surface drive value.