Prehnite only fully reveals itself as a beautiful gemstone once it has been worked. The rough material hides its potential until the polishing process brings out the distinctive pearly lustre and translucent colour that give this stone a near-luminous quality.
Prehnite derives its name from Colonel von Prehn, who discovered the gemstone in South Africa in 1788 and introduced it to Europe. It appears in yellow to mint green, pale yellow to light brown, bright yellow, grey, white and colourless. Individual gem-quality transparent crystals are rare and usually small, so remain in the domain of collectors. A fine green translucent prehnite has been marketed under the name (or misnomer) of Cape emerald.
The majority of prehnite on the market occurs naturally as a translucent mass formed as a crust or a nodule (typical of the botryoidal habit). Yellow fibrous material is also found and displays chatoyancy (a cat’s-eye effect) when cut as a cabochon. Very occasionally prehnite is pleochroic; this phenomenon is rare and increases the value of the stone. While some prehnite is translucent and clean, other material can be full of flaws and inclusions such as fractures, black tourmaline acicular (needle) inclusions and bright copper specks.
Treatments and imitations
The fine green prehnite is similar in appearance to nephrite jade and jadeite jade. It could also be confused with serpentine, apatite, chrysoprase and peridot.
Faceted stones are still viewed as collector pieces and until they become commercially available there will be no set prices. Prehnite beads and cabochons are inexpensive and readily available.
Working with prehnite
• Water is lost if prehnite is heated, and cannot be replaced. Avoid using steam cleaners and remove the stone from jewelry before soldering. Care should also be taken not to overheat the stone during polishing. Hydrochloric acid turns prehnite into a jellylike substance, so avoid contact with any chemicals.
• The cleavage of prehnite is good in one direction, and the cleavage surfaces are curved. The uneven fracture of prehnite makes it slightly brittle, so take care when setting. A ring stone should have a protective setting and its edges should not be left exposed. Prehnite is considered a good carving material, as it is soft enough to work with but can withstand reasonable wear and tear.