Prehnite

Prehnite is a calcium aluminum phyllosilicate prized for its soft, translucent yellow-green to apple-green color and characteristic grape-like botryoidal habit. It was named after Colonel Hendrik von Prehn, who brought specimens from South Africa's Cape in the late 18th century, making it one of the first minerals named after a person.

It commonly forms rounded, knobby aggregates with a slightly waxy to vitreous luster. One of its most collectible forms features dark green or black needles of epidote piercing through the pale prehnite, a combination popular with lapidaries and crystal enthusiasts.

Though relatively hard and tough, prehnite is more often cut into cabochons and beads than faceted, valued for its calming glow rather than fire.

Prehnite is a secondary mineral that forms in the cavities (vesicles) and fractures of volcanic rocks, especially basalts and related lavas, as well as in some metamorphic settings. It crystallizes from low-temperature hydrothermal fluids circulating through these rocks.

It is a classic indicator mineral of the prehnite-pumpellyite metamorphic facies, a very low grade of metamorphism. It frequently occurs alongside zeolites, calcite, datolite, and epidote, lining gas pockets left in cooling lava.

Major localities include the trap rocks of the Deccan Traps in India, South Africa (the Cape, Brandberg in Namibia nearby), Mali, Scotland, France, Australia, and the basalt quarries of New Jersey and Michigan in the United States.

Look for translucent pale green to yellow-green rounded, bumpy (botryoidal or reniform) masses with a slightly greasy or waxy luster. Internally it often shows a fibrous, radiating structure. Hardness is 6-6.5, so it scratches glass but is softer than quartz.

The streak is white and it lacks the strong cleavage of feldspars. Black or dark green needles of epidote running through the stone are a near-diagnostic giveaway.

Look-alikes include green chalcedony (harder, 7, and lacks fibrous radiating interior), jade (tougher, denser, waxier), grossular garnet (much harder, forms crystals not grape clusters), and serpentine (softer, 2.5-4). Prehnite's distinctive grape-cluster habit and pale glowing green color usually separate it.

Prehnite is mainly a collector's and lapidary stone, cut into cabochons, beads, and carvings. Transparent yellow-green material from Mali is occasionally faceted into gems. Its tough, fine-grained nature makes it suitable for jewelry, though its modest hardness means it is better in earrings and pendants than rings.

In metaphysical circles prehnite is marketed as a stone of unconditional love and calm, said to aid intuition and reduce worry; these claims are spiritual rather than scientific.

Mineralogically it is significant as a facies-defining index mineral, helping geologists gauge the temperature and pressure history of altered volcanic rocks.