Prehnite: The Bubbly Green Silicate From Basalt Cavities

Short answer: prehnite is a calcium aluminum silicate, Ca2Al2Si3O10(OH)2, orthorhombic, Mohs 6 to 6.5 and specific gravity near 2.9. It usually grows as rounded, bubbly crusts of pale mint to yellowish green in basalt cavities beside zeolites. The pieces most buyers meet come from the Kayes Region of Mali, where translucent prehnite is studded with dark epidote blades. It is untreated, inexpensive and easy to tell from dyed green look-alikes by its waxy glow and pearly luster.

What kind of mineral is prehnite?

Prehnite is a silicate of calcium and aluminum with a little water locked into its structure as hydroxyl. Webmineral gives the formula as Ca2Al2Si3O10(OH)2, a molecular weight of 395.38 and a calculated specific gravity of 2.88 (Webmineral). Some iron usually substitutes for aluminum, and the iron is what pushes the color from near white toward deeper green and yellow.

The hardness, gravity and optical figures are from the International Gem Society and Minerals.net. Hardness of 6 to 6.5 means a steel file barely marks it, but a quartz point will. That puts it in the same scratch range as feldspar and a little below agate and chrysoprase.

The name honors Colonel Hendrik von Prehn (1733 to 1785), a Dutch military commander at the Cape of Good Hope who brought specimens to Europe. Webmineral records prehnite as a pre-IMA species described in 1789,.

How does prehnite form?

Prehnite is a low-temperature mineral. The International Gem Society summarizes its occurrence as deposition from ground waters in basaltic rocks associated with zeolites, plus hydrothermal crystals in cavities of acid igneous rocks and late-stage growth in serpentine rocks. In plain terms: hot, calcium-rich fluids seep through cooled lava flows, and prehnite precipitates in gas pockets and fractures at temperatures well below those that make most gem silicates.

That shared birthplace is why prehnite turns up with the zeolites. Minerals.net lists its common associates as calcite, natrolite, stilbite, pectolite, epidote, babingtonite, laumontite, chabazite and apophyllite. If you collect Indian zeolites, you have probably seen pale green prehnite crusts under apophyllite or stilbite crystals.

What do the shapes mean?

Single, sharp prehnite crystals are rare. Minerals.net describes the usual habit as rounded, bubbly globular masses built from small saddle-shaped crystals or radiating balls, and lists botryoidal, reniform, encrusting, stalactitic and acicular forms. Look at the surface of a "bubble" with a loupe and you can often see the edges of the tiny curved crystals that make it up.

Prehnite also grows over other minerals and outlasts them. It commonly forms epimorphs after anhydrite, glauberite and laumontite: it coats a crystal, the crystal later dissolves, and a hollow prehnite cast is left behind. Collectors call the elongated ones "prehnite fingers", and an elongated cast with a widened head a "snakehead". These are prized classics from the New Jersey traprock quarries.

Where does prehnite come from?

Mali: prehnite with epidote

The material on most shop tables today is African. According to Minerals.net, the most extensive and exploited deposits are in the Kayes Region of Mali, where prehnite forms thick, translucent masses associated with sharp dark epidote. Named Kayes localities include Djouga (Bendoukou), Diakon, Sandare and Sadiola. The epidote occurs as dark, chisel-shaped crystals beside or inside the prehnite, and in polished pieces they read as black or deep green needles and blades suspended in pale green.

Those needles are the selling point of "prehnite with epidote". Epidote is a separate mineral, Ca2(Al,Fe)2(SiO4)3(OH), monoclinic, hardness 6 to 7 and specific gravity 3.3 to 3.6 (Minerals.net on epidote). It is denser and darker than its host, and it polishes to a brighter, more glassy spot.

Australia

Minerals.net cites good prehnite from Prospect and Mullaley in New South Wales, and pictures finger-shaped pieces from Tambar Springs in the same state. The International Gem Society notes that yellowish to greenish translucent Australian material has been faceted into gems with a soft, velvety look, sometimes up to about 30 carats, and that rare cat's-eye stones have been reported. Australian prehnite tends to be more yellow than Malian.

New Jersey, Scotland and elsewhere

The traprock quarries of Passaic County, New Jersey, especially the Upper and Lower New Street Quarries in Paterson and Prospect Park, produced some of the finest botryoidal masses and fingers known. The International Gem Society records masses up to several tons in New Jersey traprocks, and single masses of 400 pounds collected. Scotland has given small facetable fragments, typically under 5 carats. Other notable sources listed by Minerals.net include Asbestos, Quebec (colorless crystals), the Merelani Hills of Tanzania (highly transparent material), the Goboseb Mountains of Namibia (mint green), and the N'Chwaning mines of South Africa, where prehnite occurs as rare bright orange needles.

How do you recognize prehnite?

Four things, in order:

1. Color and glow. Pale mint, apple or yellow-green with a milky, translucent glow, as if lit from inside. Minerals.net's gemstone page notes that deep green prehnite is not common and that it often looks cloudy or velvety.
2. Surface. Raw pieces are bubbly or have rows of curved, saddle-like crystal edges with a pearly sheen. Polished pieces keep a waxy luster.
3. Inclusions. Black or dark green epidote blades are a strong pointer to Malian prehnite.
4. Weight. At about 2.9 it feels slightly heavier in the hand than an agate of the same size (about 2.6), though less so than jade.

What is prehnite confused with?

• Chrysoprase. Nickel-colored green chalcedony, with its green usually caused by traces of nickel (Geology.com). It is a more saturated apple green and lacks prehnite's pearly crystal texture. For a deeper side-by-side with jade, see chrysoprase vs jade.
• Dyed green chalcedony or agate. Geology.com notes that a lot of ordinary chalcedony and agate is dyed green to imitate chrysoprase; the same dyed beads are sometimes passed off as prehnite. Dye looks too even and gathers in cracks and drill holes.
• Jade. Minerals.net's gemstone page says jade is usually deeper in color and less transparent than prehnite.
• Peridot. Slightly harder and usually more transparent.
• Smithsonite and hemimorphite. Both can form similar pale botryoidal crusts; smithsonite is softer and fizzes in hydrochloric acid, and hemimorphite is heavier and softer.
• Green glass. Glass has no crystal structure on the surface, may show round bubbles, and feels lighter for its look.

Is prehnite treated?

Mostly no. Minerals.net's gemstone section states that prehnite gemstones are natural and not heated or artificially enhanced. Two exceptions apply to raw specimens. Prehnite sometimes carries a brown iron coating that some sellers strip with chemicals, and some dealers oil specimens with lubricants to enhance luster. A specimen that looks wet and greasy, or leaves a film on your fingers, has probably been oiled. Oil can be removed with warm soapy water, but the luster you paid for leaves with it.

How should you care for prehnite?

Treat it as moderately tough but not robust. It is hard enough for pendants and earrings, but the distinct cleavage and brittle bubbles mean a hard knock can flake a botryoidal crust. Minerals.net warns that prehnite is very sensitive to heat and chips easily during cutting, so keep it away from jewelers' torches, ultrasonic cleaners and hot water. Clean with lukewarm water, a drop of mild soap and a soft brush, then let it air dry. Store raw pieces in a padded tray where nothing can roll into the bubbles.

What is prehnite associated with in crystal traditions?

In crystal traditions, prehnite is associated with calm intuition and with connecting the heart to nature, and it is sometimes paired with epidote as a "growth" combination. These are beliefs, not physical properties.

What to look for when you buy prehnite

Prehnite is an inexpensive stone. Minerals.net calls it an inexpensive gemstone, usually cut into cabochons and beads because it is rarely fully transparent. Price is driven by translucency, even color, the pattern of epidote, and, for raw pieces, intact bubbles and a clean crystal surface.

• Translucency first. Hold a piece up to a window. Good prehnite glows through; dull, chalky pieces are the bottom grade.
• Epidote pattern. In Malian material, look for epidote blades that sit cleanly inside the stone rather than broken, crumbly patches on the surface.
• Intact surfaces. On raw pieces, check the tops of the bubbles for chips and bruises, which show as white spots.
• No grease. A clean, dry luster is natural; a slick, oily one may mean an oil treatment.

Our current stock:

• Prehnite, small piece: the entry-level option, a good way to see prehnite's pale green, waxy translucency up close for very little money.
• Prehnite, larger piece: a bigger, better-glowing example; check the listing photos for visible epidote blades and an even mint color.
• Epidote Crystal Clusters on Matrix: a 5 by 2.5 by 2 inch, 273 g cluster of epidote on its own. Set it beside a Malian prehnite to see what those dark needles look like as free-grown crystals.

Frequently asked questions

What are the black needles in prehnite?

In most prehnite from the Kayes Region of Mali, the dark needles and blades are epidote, a calcium aluminum iron silicate. Epidote is denser than prehnite and much darker, so it shows as black or very dark green crystals suspended in the pale green host. They are natural and expected.

Is prehnite a rare stone?

No. Prehnite is common in basalt cavities worldwide, and large African deposits keep it inexpensive. Rarities do exist: fully transparent facet-grade crystals, cat's-eye stones from Australia, sharp individual crystals, and the orange needle form from South Africa command much higher prices.

Can prehnite go in water?

Brief rinsing in lukewarm water is fine; prehnite is not water soluble. Avoid hot water, long soaks with oiled or coated specimens, and ultrasonic or steam cleaners, because prehnite is heat sensitive and has a distinct cleavage that can open under shock.

How can you tell prehnite from chrysoprase?

Chrysoprase is a nickel-colored chalcedony with a more saturated apple green and a smooth, even texture. Prehnite is paler and more yellow or mint, with a pearly crystal texture on raw surfaces, curved crystal edges in its bubbles, and often dark epidote needles.

Is prehnite with epidote a separate stone?

No. It is prehnite that grew together with epidote crystals, mainly in Mali. Some sellers market the combination as if it were its own variety, but it is two distinct minerals in one piece, each with its own formula and properties.

Sources

• Minerals.net, The mineral prehnite: https://www.minerals.net/mineral/prehnite.aspx
• Minerals.net, The gemstone prehnite: https://www.minerals.net/gemstone/prehnite_gemstone.aspx
• International Gem Society, Prehnite Value, Price, and Jewelry Information: https://www.gemsociety.org/article/prehnite-jewelry-and-gemstone-information/
• Webmineral, Prehnite Mineral Data: http://webmineral.com/data/Prehnite.shtml
• Minerals.net, The mineral epidote: https://www.minerals.net/mineral/epidote.aspx
• Geology.com, Chrysoprase: https://geology.com/gemstones/chrysoprase/