Prehnite

Prehnite is a silicate mineral containing calcium and aluminium, mostly pale green in colour. Instead of distinct crystals it usually forms kidney-shaped, grape-like or globular clusters. It is named after Colonel Hendrik von Prehn, commander of the Dutch colony in South Africa in the 18th century. Translucent green material is cut as cabochons and beads, and the rare transparent pieces as faceted stones.

Jewellery: Translucent green material is worked into cabochons, beads and small carvings. Transparent, clean pieces are cut as faceted stones; these are sometimes sold as “Cape emerald”. Decoration and collecting: Rounded, pale green clusters and well-formed crystals are sought after by collectors; it is also sold as tumbled stones.

It has a hardness of 6–6.5 on the Mohs scale. It can scratch glass; a steel knife scratches it only with difficulty. Its specific gravity is 2.8–2.95 g/cm³.

Its best-known localities: Cradock area, Eastern Cape, South Africa; Wave Hill Station area, Northern Territory, Australia; Le Bourg-d'Oisans (La Combe-de-la-Selle), Isère, France; Paterson, New Jersey, USA; Kalahari manganese field (Hotazel area), Northern Cape, South Africa.

Its colour runs from pale green to dark green and yellowish green; white, grey, yellow, pink, blue and, rarely, orange examples also occur. Its lustre is glassy to slightly pearly. It mostly occurs not in distinct crystals but as kidney-shaped, grape-like, stalactitic or globular clusters; well-formed crystals with a square cross-section are very rare. Its hardness is 6–6.5; it is brittle and breaks unevenly. It is mostly translucent and only rarely transparent.

On the “street to diamond” value ladder it sits on step 5 of nine: Ornamental stone. What matters most in prehnite is its colour and translucency: vivid, even green and clean, translucent material is sought after for cabochons and beads. Transparent stones clean enough to be faceted are rare. Crystal clusters and attractive rounded specimens are valued by collectors. The extensive deposits around Wave Hill in Australia supply jewellery-grade material.

Today crystal-therapy practitioners regard prehnite as a “stone of unconditional love”, link it to the heart and solar plexus chakras, and claim that it calms anger and brings peace, dissolves repressed memories and unpleasant feelings, and reduces nightmares, phobias and deep fears. The same circles claim that prehnite helps the health of bones, teeth, hair and nails, the circulation, the kidneys and the lungs, helps the body get rid of fats and toxins, and helps in gout and blood disorders. There is no scientific evidence that prehnite calms emotions, reduces fears or helps bone, circulatory, kidney or lung health; conditions such as gout, blood disorders or lasting anxiety call for medical assessment. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.