Rock Identifier
Prehnite (Calcium aluminum silicate hydroxide - Ca2Al(AlSi3O10)(OH)2) — mineral
mineral

Prehnite

Calcium aluminum silicate hydroxide - Ca2Al(AlSi3O10)(OH)2

Hardness: 6-6.5 (Mohs scale); Color: Pale green to yellow-green, often translucent; Luster: Vitreous to pearly; Crystal structure: Orthorhombic, typically occurring in botryoidal (grape-like) or mammillary habits; Cleavage: Good in one direction; Specific Gravity: 2.80 - 2.95.

Hardness
6-6
Color
Pale green to yellow-green, often translucent
Luster
Vitreous to pearly
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Physical properties

Hardness: 6-6.5 (Mohs scale); Color: Pale green to yellow-green, often translucent; Luster: Vitreous to pearly; Crystal structure: Orthorhombic, typically occurring in botryoidal (grape-like) or mammillary habits; Cleavage: Good in one direction; Specific Gravity: 2.80 - 2.95.

Formation & geological history

Formed as a secondary or hydrothermal mineral in cavities, veins, and amygdales of mafic volcanic rocks (like basalt). It typically forms from low-grade metamorphic processes.

Uses & applications

Primarily used as a semi-precious gemstone for jewelry (cabochons and beads) and highly valued by mineral collectors for its unique habit and 'inner glow' appearance.

Geological facts

Prehnite 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 referred to as 'Cape Chrysolite'.

Field identification & locations

Identify in the field by its distinct oil-green color, translucency, and occurrence inside volcanic gas bubbles (vesicles). Found commonly in South Africa, Australia, Scotland, and the USA (New Jersey). Collectors should look for rare epimorphs where prehnite has coated and then replaced other minerals like laumontite.

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