Prehnite is a calcium aluminum silicate mineral prized for its translucent, pale yellow-green color and soft inner glow. It usually forms rounded, bubbly masses instead of distinct crystals, so cutters shape it into cabochons and beads. Material from Australia and Mali is common in jewelry, and some stones carry dark needles of epidote that add contrast.
What prehnite is
Prehnite has the formula Ca2Al2Si3O10(OH)2. It is a silicate built in sheets, a structure that gives it a slightly pearly luster on broken surfaces. Its typical colors are pale green, yellow-green and greenish yellow, with some white, gray and brown material. Iron replacing aluminum in its structure gives it the green tint.
Prehnite is often cited as the first mineral named after a person. It honors Hendrik von Prehn, a Dutch military officer who brought samples from the Cape of Good Hope in South Africa in the 18th century. Mineralogists described it formally in 1788.
Crystal habit and appearance
Individual prehnite crystals are uncommon and usually small. Most prehnite grows as botryoidal masses, which look like clusters of grapes. It also forms crusts, fan-shaped sprays and stacked plates that resemble a half-opened book. The page on crystal habit explains these terms and how habit helps identify rough material.
The translucency gives prehnite a slightly hazy glow, sometimes compared to the look of green grapes or jade. Good material is evenly colored and free of visible cracks. Some transparent prehnite has been faceted for collectors, but cabochons show its color and glow best.
Epidote inclusions
Prehnite from Australia and Mali often contains needles or sprays of epidote, a dark green to black mineral. The needles stand out against the pale green host. Cutters orient cabochons to show them well, and many buyers seek these stones. Malian material often has black epidote crystals radiating through or alongside it.
Other inclusions include tiny native copper grains in prehnite from the Lake Superior region of Michigan, where prehnite occurs with copper in old lava flows.
Where prehnite forms
Prehnite usually forms in cavities and veins in volcanic rocks such as basalt. Hot, mineral-rich water circulates through the rock after the lava cools and deposits prehnite along with zeolites, calcite and quartz. It also forms in some low-grade metamorphic rocks. The igneous rocks and gemstones page covers basalt and its cavity minerals.
Sources include:
- Australia, with gem material from Wave Hill in the Northern Territory and other localities.
- Mali, which produces yellow-green prehnite with black epidote needles, mostly from the Kayes region.
- South Africa, the original locality.
- China, the United States (New Jersey, Michigan and Virginia), Scotland and France.
Properties and identification
Prehnite's properties help a gemologist separate it from green look-alikes. The table lists the usual ranges.
| Property | Typical value |
|---|---|
| Refractive index | 1.611-1.669 |
| Birefringence | 0.020-0.035 |
| Specific gravity | 2.80-2.95 |
| Mohs hardness | 6-6.5 |
| Optic character | Biaxial positive |
On a refractometer, cabochons give a single spot reading near 1.63, because the curved surface does not show both edges of the birefringence. The fibrous, radiating structure of botryoidal masses can also give some stones a faint silky sheen, and a few show a weak cat's eye effect when cut with the fibers parallel to the base. The page on the refractive index of gemstones explains spot readings and why cabochons behave this way.
Prehnite in jewelry
Prehnite appears mostly in cabochons, beads, briolettes and carved pendants. It pairs well with yellow gold and silver and has an understated look. Because it is affordable and widely available, it is often used in larger pieces.
Look-alikes include chalcedony, chrysoprase, green jade, serpentine and green glass. Prehnite has a higher refractive index and density than chalcedony, which a gemologist can measure. Jadeite and nephrite are much tougher. The page on jade explains how jade differs from look-alikes.
Prehnite is rarely treated. Some material is dyed or impregnated with resin, and sellers should disclose this.
Choosing prehnite
Look for even color across the stone and a smooth, glassy polish. Translucent stones with a slight glow are more attractive than cloudy or chalky ones. In stones with epidote, the needles should be well placed and should not reach the surface in a way that leaves pits. Cracks along the botryoidal growth layers are a common weak point, so check the edges and the back of a cabochon under good light.
Collectors also value prehnite as a mineral specimen. The basalt quarries of northern New Jersey have produced pale green prehnite with zeolites such as stilbite and natrolite, and Mali has supplied plates of prehnite studded with dark epidote crystals.
Durability and care
Prehnite has a hardness of 6 to 6.5, softer than quartz dust. Its cleavage is rarely a problem in everyday wear and good toughness for its hardness. It handles pendants and earrings well. In rings, use a protective bezel and remove the ring for rough work.
Clean prehnite with warm soapy water and a soft cloth. Avoid ultrasonic and steam cleaners, sudden temperature changes and household chemicals. The guide to cleaning jewelry safely covers general cleaning, and gemstone durability compares wearability across gems.
Frequently asked questions
What color is prehnite?
Prehnite is usually pale yellow-green to green. White, gray, yellow and brownish pieces also occur.
Is prehnite rare?
No. Prehnite is fairly common as a mineral. Clean, evenly colored gem material is less common but still widely available.
Can prehnite get wet?
Short contact with water is fine, and cleaning with mild soap and water is safe. Avoid long soaks, hot water and chemicals.
What are the black needles in prehnite?
They are usually epidote, a separate mineral that grew alongside the prehnite. They are natural inclusions.