Gem library
Prehnite
The grape green of lava flows
Prehnite is a calcium-aluminium silicate of a characteristic milky apple green, lining cavities in volcanic rock as botryoidal crusts that look like bunches of grapes. It also holds a particular place in the history of mineralogy.
- Hardness (Mohs)
- 6 – 6.5
- Density
- 2.80 – 2.95
- Crystal system
- Orthorhombic
- Refractive index
- 1.611 – 1.669
- Cleavage
- Distinct on {001}
Mineralogical identity
- Family
- Prehnite — phyllosilicate group
- Species
- Prehnite
- Chemical formula
- Ca₂Al₂Si₃O₁₀(OH)₂
- Crystal system
- Orthorhombic (botryoidal aggregate)
- Classification
- Semi-precious stone
- Rarity
- Common
- Indicative price
- €3 – 25/ct
Etymology
Named in 1789 after Colonel Hendrik von Prehn, governor of the Cape colony, who brought the first specimens to Europe. It is the first mineral in history to bear a person's name.
Physical & optical properties
- Hardness (Mohs)
- 6 – 6.5
- Density
- 2.80 – 2.95
- Refractive index
- 1.611 – 1.669
- Birefringence
- 0.020 – 0.033
- Cleavage
- Distinct on {001}
- Fracture
- Uneven
- Lustre
- Vitreous to pearly
- Diaphaneity
- Translucent to transparent
- UV fluorescence
- None
- Pleochroism
- Weak
- Colours
- green, yellow, apple-green, colourless
Formation, geology & history
Formation and habit
Prehnite is a secondary mineral: it is deposited in the vesicles and fissures of basalts and dolerites as hot alkaline water circulates after the lava cools. It rarely forms isolated crystals, but almost always mammillated crusts, spheres and «cauliflowers» made of thousands of small radiating tabular crystals. These botryoidal, translucent, softly green forms made its reputation. The Diakon deposit in Mali, worked since the 1990s, yields perfectly round apple-green spheres, often hollow and lined with black epidote, which appear in every collection.
A place in the history of science
Before 1789, minerals bore descriptive names — haematite, the bloody one; malachite, the mallow-coloured one — or place names. Abraham Gottlob Werner broke with custom by naming this one after Colonel von Prehn, opening the way to a practice that now dominates mineralogical nomenclature: goethite, kunzite, morganite, alexandrite. Prehnite is thus, without intending it, the ancestor of a tradition that now counts several thousand species named after people.
As gem and as cabochon
Translucent prehnite cuts into cabochons of a very soft milky green, often compared to soft jade or pale chrysoprase. Genuinely transparent material, rarer, is faceted and yields luminous yellow-green stones, notably in Australia and China. Hardness of 6 to 6.5 and distinct cleavage call for a minimum of care: it suits pendants and earrings well, heavily worn rings less so. Its modest price, a few euros a carat, makes it accessible in large volumes, and it is widely used in beads and bracelets.
How to recognise it
- The grape-cluster formIn the rough, the mammillated cauliflower or translucent sphere habit is decisive: no other common green stone presents itself this way. Broken, it reveals a radiating fibrous structure.
- Milky pearly greenIts colour is a veiled apple green, never clear or saturated, with a slight pearly sheen on cleavage surfaces. A vivid transparent green suggests chrysoprase or apatite instead.
- Hardness and cleavageIt scratches glass but is marked by quartz. The pearly cleavage planes, visible under the loupe on fractures, distinguish it from chalcedonies, which have none.
Imitations, treatments & confusions
- Chrysoprase and «jade» sold as prehniteAnd the reverse. Chrysoprase is a harder chalcedony (6.5-7) with no cleavage; serpentine is softer and waxy. Prehnite's radiating structure and pearly sheen betray it under the loupe.
- Opalescent green glassHomogeneous, with round bubbles, no fibrous structure or cleavage. It lacks that veiled-from-within look so typical of prehnite.
- Green resin beadsFrequent in cheap bracelets. Far lighter, warm to the touch, and scratches with a fingernail. Weight in the hand alone gives it away.
Deposits & origins
Mali (Diakon, Kayes — the celebrated apple-green spheres), Australia (Wave Hill), China (Sichuan), Scotland (Kilpatrick Hills, Dunbartonshire), United States (Paterson, New Jersey), South Africa (the Cape type locality), India (Poona), France (Vosges, Massif Central)
Care & precautions
- Warm soapy water and a soft cloth; brief rinse and careful drying, without prolonged soaking.
- Avoid ultrasonic cleaning: distinct cleavage and radiating structure do not appreciate vibration.
- Do not leave it long in the sun, which can dull the green, and avoid acids.
- Store away from quartz and hard stones, which would scratch it easily.
Traditions & crystal healing
- Chakras
- Heart · solar plexus
- Zodiac signs
- Libra · Virgo
- Element
- Earth · Water
- Month
- May
Crystal healing readily calls it the «stone of prophecy» and links it to the heart and the plexus: it is said to favour inner tidying, mental decluttering and preparation for action. Often recommended alongside epidote, with which it naturally occurs, it is presented as a stone of renewal and of putting things in order.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Prehnite or jade?
Two unrelated minerals. Jade — jadeite or nephrite — is far tougher and denser, with no workable cleavage. Prehnite is softer, cleaves distinctly, and its green is milkier and yellower.
Can it go in water?
Brief cleaning, yes. Avoid long soaking and salt water: the radiating structure and cleavage make the surface sensitive, and the polish dulls.
Why is it so cheap?
Because it is abundant: prehnite lines basalt flows on every continent, and the Malian deposits produce large quantities. It is a beautiful common stone, not a rarity.