Prehnite
Prehnite
The yellow-green calcium-aluminium silicate of basaltic vugs and zeolite assemblages
Prehnite is a calcium aluminium silicate hydroxide mineral with the formula Ca2Al2Si3O10(OH)2, encountered most often as translucent yellow-green to apple-green material with a pearly to vitreous lustre. It is found in the cavities of basaltic and andesitic volcanic rocks, often in association with the zeolite group, and occurs occasionally in metamorphic settings. Prehnite is hard enough at 6 to 6.5 on the Mohs scale to take a respectable polish, and is most often cut en cabochon or carved; transparent crystals suitable for faceting are uncommon and command a premium when found.
Mineralogy and properties
Prehnite is orthorhombic, with refractive indices of approximately 1.611 to 1.669 and a specific gravity around 2.80 to 2.95. The colour range runs from pale yellow through yellow-green to apple-green, with grey, white, and rare colourless to pinkish material also recorded. The colour-producing chromophore is principally iron, with the saturation of the green tone tracking iron content. The mineral is commonly fibrous to crystalline-massive in habit, with botryoidal and stalactitic forms occurring in vug-fill specimens. The fibrous habit is responsible for the chatoyancy occasionally seen in cabochons cut from selected material.
Prehnite is named for the Dutch colonel Hendrik von Prehn, who introduced specimens to Europe from the Cape of Good Hope in the late eighteenth century. The mineral is one of the few species named for the person who collected it rather than for a property of the material itself.
Origin and sources
Prehnite forms during the late stages of basaltic volcanism, crystallising in vugs and amygdules from low-temperature hydrothermal fluids alongside zeolite-group minerals such as natrolite, stilbite, and apophyllite. The mineral is also found in metamorphic rocks of the prehnite-pumpellyite facies, a low-grade regional metamorphic regime named for the species. Major commercial sources include Australia (notably the Wave Hill area of the Northern Territory), South Africa (the type locality area in the Eastern Cape), Mali, Namibia, and China.
Australian and Malian prehnite is most associated with the rich, saturated apple-green material that commands the highest prices in the cutting trade. Chinese material tends toward yellower tones, and South African material covers the full colour range from pale to deeply saturated.
Cutting and trade
The standard product is the cabochon — a domed cut that brings out the slightly waxy, translucent character of the material and the chatoyancy where present. Carved prehnite, often in beads and Asian-market figurative pieces, accounts for a significant share of commercial output. Faceted transparent prehnite, where it occurs, is most often produced in fancy cuts that maximise face-up colour from modest-saturation rough; the material's modest refractive index does not reward brilliant cutting in the way that higher-index species do.
Prehnite is sometimes confused at retail with chrysoprase or with serpentine. The hardness, the higher specific gravity, and the characteristic pearly lustre on broken surfaces distinguish it from both. Imitations are uncommon — the species has not historically attracted the synthesis effort directed at higher-value materials.
In the trade
Prehnite has a steady but modest market presence as an attractive semi-precious cabochon and bead material. The most desirable material is transparent to highly translucent, with strong apple-green saturation and minimal grey overtone. Cat's-eye prehnite — chatoyant cabochons with a clean light band against a translucent green body — commands a notable premium in the connoisseur cabochon market. The species is rarely encountered in mainstream commercial jewellery and is more often a designer or artist-jeweller selection.
Care
At hardness 6 to 6.5, prehnite is suitable for pendants, earrings, and protected ring designs but is too soft for unprotected daily-wear ring use. Cleaning is by mild soap and warm water with a soft brush. Ultrasonic and steam cleaning are not recommended, as the species can show fluid inclusions and fibrous structure that may respond poorly to vibration or thermal shock.