Prehnite

Hardness: 6-6.5 (Mohs), Color: Pale green to apple-green (most common), yellowish-green, gray, white; often translucent to subtranslucent. Luster: Vitreous to pearly on cleavage surfaces. Crystal Structure: Orthorhombic, often forming botryoidal (grape-like), reniform (kidney-shaped), or mammillary aggregates. Individual crystals are rare and typically tabular or prismatic. Cleavage: Good on {001}. Specific Gravity: 2.80-2.95.

Prehnite is a secondary mineral, typically forming in the veins and cavities of mafic igneous rocks (like basalt, gabbro, and diabase) during low-grade metamorphic conditions (zeolite or prehnite-pumpellyite metamorphism). It forms from hydrothermal alteration of plagioclase feldspar and other calcium-aluminum silicates. Geological Age: Varies depending on the host rock, but generally associated with later-stage alteration processes.

Uses: Primarily as a gemstone for jewelry, often cut into cabochons, beads (as seen in the image), or carvings. Its attractive green color and translucent nature make it popular. Also collected by mineral enthusiasts. It is not used in industry or construction due to its relative softness and the small quantities in which it forms.

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 found associated with zeolites, datolite, and calcite. The botryoidal habit is particularly characteristic and sought after by collectors.

Identification in the field: Look for its distinct pale to apple-green color and characteristic botryoidal or globular habit filling cavities or veins in mafic igneous rocks. It can be distinguished from jade by its lower hardness (jade is typically 6.5-7) and different crystal habit. Common locations: South Africa (especially the Karoo Basin), Australia (New South Wales), USA (New Jersey), Scotland, China, India, and France. Tips for collectors: Look for well-formed botryoidal aggregates or clear, vibrant green crystals. Specimens with good translucency are highly valued. Be aware of synthetic imitations or look-alikes that may be sold as prehnite.