mineral

Prehnite

Prehnite, Ca2Al(AlSi3O10)(OH)2

Hardness: 6-6.5 (Mohs scale), Color: Pale green to yellow-green, Luster: Vitreous to pearly, Crystal Structure: Orthorhombic (often occurs in botryoidal or globular habits), Cleavage: Distinct in one direction, Specific Gravity: 2.80 - 2.95

Hardness: 6-6

Identified May 25, 2026More mineral →

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Physical properties

Formation & geological history

Formed in low-grade metamorphic environments and hydrothermal veins, typically as a secondary mineral in the cavities of mafic volcanic rocks like basalt. Many specimens date from the Mesozoic era.

Uses & applications

Primarily used as a gemstone for jewelry (cabochons and beads) and as a popular collector's mineral specimen. Historically used for minor ornamental carvings.

Geological facts

It was the first mineral to be named after a person, Colonel Hendrik Von Prehn, who discovered it in South Africa in 1788. It is sometimes called 'Cape Emerald' although it is not an emerald.

Field identification & locations

Identify by its characteristic 'seafoam' green color, greasy luster, and common botryoidal (grape-like) formation. Found commonly in South Africa, Australia, and the United States (New Jersey/Connecticut). Collectors look for clarity and the presence of epidote inclusions.