Prehnite Identification Guide
A practical field guide to identifying prehnite by its botryoidal apple-green crusts, hardness, and basalt-vug setting.
Read the full Prehnite encyclopedia entry →
What Prehnite Looks Like
Prehnite is a calcium-aluminum silicate (Ca2Al2Si3O10(OH)2) best known for its soft, glowing apple-green to yellow-green color, though it also occurs colorless, white, gray, and bluish-green. Its luster is vitreous to slightly waxy or pearly, and it is typically translucent with a slightly cloudy, gel-like interior.
The most telling feature is its habit: prehnite almost always forms botryoidal (grape-like), reniform, or stalactitic crusts lining cavities. Broken surfaces reveal a fine radiating fibrous structure. Well-formed tabular crystals exist but are far less common than the rounded massive crusts.
Step-by-Step Field ID Checklist
- Color and glow: Look for a translucent green to yellow-green stone with an internal "lit-from-within" softness.
- Surface shape: Check for rounded botryoidal bumps or stalactitic forms in a rock cavity.
- Examine a chip: Note the radiating fibrous internal texture.
- Hardness test: It should scratch glass but be scratched by quartz.
- Setting: Confirm it sits in vugs/amygdules of basalt, often alongside zeolites, calcite, datolite, or pectolite.
Key Diagnostic Tests
- Mohs hardness: 6–6.5 (scratches glass; quartz scratches it).
- Streak: white.
- Cleavage: good on {001}, but rarely visible in the typical massive material; fracture is uneven.
- Specific gravity: ~2.8–2.95 (noticeably heavier than common quartz of similar size).
- Acid: no reaction to dilute HCl (it is a silicate, not a carbonate — separates it from calcite).
Common Look-Alikes and How to Tell Them Apart
- Nephrite jade: very tough and resists chipping; prehnite is more brittle and shows a fibrous radiating fracture rather than jade's felted toughness.
- Chrysoprase / green chalcedony: harder (Mohs 7), waxier, and lacks any cleavage or radiating fiber.
- Hydrogrossular garnet: harder (~7) and granular rather than botryoidal.
- Green opal: lower hardness (5.5–6.5), no radiating fibers, and an even resinous look.
- Serpentine: softer (3–5), greasy feel, easily scratched by a knife.
A combination of botryoidal habit, hardness 6–6.5, no acid reaction, and the radiating fibers reliably confirms prehnite.
Where Prehnite Is Found
Prehnite forms in cavities of basalt and other mafic volcanic rocks during low-grade metamorphism and hydrothermal activity. Classic sources include the Karoo basalts of South Africa (where it was first described), Australia, Mali, China (which produces grape-green carved material), Scotland, and the trap-rock quarries of New Jersey, USA.
Frequently asked questions
How can you tell if it's real prehnite?
Look for a translucent apple-green stone with botryoidal (grape-like) bumps, a hardness of 6–6.5 (it scratches glass), no fizz in acid, and a fine radiating fibrous texture on broken surfaces.
What does prehnite look like?
It is usually a soft, glowing yellow-green to apple-green translucent material forming rounded botryoidal or stalactitic crusts, with a vitreous-to-waxy luster and a slightly cloudy, gel-like body.
Prehnite vs jade — how are they different?
Nephrite jade is extremely tough and hard to chip, while prehnite is more brittle and shows radiating fibers and botryoidal surfaces. Prehnite is also usually more translucent and lower in density.
Is prehnite the same as green opal?
No. Green opal is softer (5.5–6.5), has an even resinous look, and lacks the radiating fibrous structure and botryoidal crusts typical of prehnite.
Prehnite identified by the community
Recent Prehnite specimens identified with Rock Identifier.
