Gem library
Apophyllite
Glass pyramids from the Deccan
Apophyllite forms pyramids and prisms with the transparency of glass, often sitting on a bed of pink stilbite or chalcedony. The finest specimens come from the Deccan quarries in India, where geodes sixty-five million years old are opened by accident.
- Hardness (Mohs)
- 4.5 – 5
- Density
- 2.30 – 2.40
- Crystal system
- Tetragonal
- Refractive index
- 1.530 – 1.537
- Cleavage
- Perfect on {001}
Mineralogical identity
- Family
- Silicates, phyllosilicates
- Species
- Fluorapophyllite (series)
- Variety
- Green apophyllite, colourless
- Chemical formula
- KCa₄Si₈O₂₀(F,OH)·8H₂O
- Crystal system
- Tetragonal
- Classification
- Collector's mineral
- Rarity
- Uncommon
- Indicative price
- €0.20 – 5/ct
Etymology
From the Greek apo, «away from», and phyllon, «leaf»: heated, the stone unleafs and exfoliates, which gave it its name in 1806.
Physical & optical properties
- Hardness (Mohs)
- 4.5 – 5
- Density
- 2.30 – 2.40
- Refractive index
- 1.530 – 1.537
- Birefringence
- 0.002
- Cleavage
- Perfect on {001}
- Fracture
- Uneven
- Lustre
- Vitreous to pearly on the cleavage
- Diaphaneity
- Transparent to translucent
- UV fluorescence
- Weak
- Pleochroism
- None
- Colours
- colourless, white, green, pale green, pink
Formation, geology & history
Formation and geology
Apophyllite belongs to the zeolite company, those minerals that line the cavities of volcanic rocks. It forms at low temperature, when alkaline waters move through the voids left by gas bubbles in a lava flow. The Deccan traps in India are the absolute example: two million cubic kilometres of basalt erupted at the end of the Cretaceous, riddled with cavities in which apophyllite, stilbite, heulandite and chalcedony developed. The quarries at Poona and Jalgaon, worked for road ballast, yield by accident the geodes that mineralogists the world over fight over.
One family, three names
What the trade calls «apophyllite» is not a single species but a series of three: fluorapophyllite, hydroxyapophyllite and natroapophyllite, differing in the dominant anion or cation. The vast majority of specimens are fluorapophyllite, which is why the simple name is still used everywhere. The mineralogical nomenclature commission did in fact make «apophyllite» invalid as a species name in 2013, which changed absolutely nothing on shop labels. The green tint, the most sought after, comes from vanadium.
Transparency and fragility
Colourless apophyllite reaches a clarity rare in so soft a mineral, and its pyramid faces often have a perfect natural polish. That perfection has a price: a perfect basal cleavage means a crystal peels away in plates under a fingernail, and the stone holds eight water molecules per formula unit, which it loses if heated. A specimen left near a radiator dulls and eventually flakes. Faceted crystals exist but belong to the realm of collector's curiosities: at hardness 4.5 and with that cleavage, no worn stone would survive.
How to recognise it
- The tetragonal pyramid shapeA square prism capped by a four-sided pyramid: that is the classic silhouette, very different from quartz's six faces. A hexagonal crystal is not apophyllite.
- Pearly cleavageOn the base, the perfect cleavage returns a pearly, almost iridescent lustre, clearly distinct from the vitreous lustre of the side faces. The contrast is visible to the naked eye.
- Hardness 5A steel point scratches it readily, glass barely resists it. A transparent crystal that steel cannot scratch is quartz, not apophyllite.
Imitations, treatments & confusions
- Quartz sold as apophylliteAn honest and frequent confusion on colourless pieces. Quartz has six faces and scratches glass; apophyllite has four and is scratched by steel.
- Reglued specimensBroken tips are glued back with cyanoacrylate on pieces for sale. Look for too clean a joint at the base and a glossy excess of glue under a loupe.
- Reconstituted matrixDetached crystals remounted on a false base of tinted plaster. Discreetly scratch the base under the piece: basaltic rock is hard, plaster is not.
Deposits & origins
India (Poona and Jalgaon, in the Deccan basalts: the world reference), Brazil, Iceland, Mexico, United States (New Jersey), Scotland, Germany
Care & precautions
- Never water: the crystals come loose from their matrix and the cleavage holds the moisture.
- Dust with a soft brush, blowing rather than rubbing.
- Away from any heat: the stone loses its structural water and dulls permanently.
- A closed cabinet for preference, the tips break at the slightest handling.
Traditions & crystal healing
- Chakras
- Crown · third eye
- Zodiac signs
- Gemini · Libra
- Element
- Air
It is associated with clarity and openness, and is readily placed in a room where one works. Its name of «truth stone» comes from its transparency, not from a property. Cultural heritage, never medicine.
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
Why has my apophyllite turned milky?
It has dried. The eight water molecules in its formula leave with heat, and the surface clouds. The process is irreversible: keep it away from radiators and from full summer sun behind glass.
Green and colourless apophyllite, the same mineral?
Yes, the green comes from traces of vanadium. Green material is more sought after and comes mainly from Jalgaon, in India.
Can it be cleaned with water?
No. On a specimen on matrix, water loosens the crystals and leaves marks in the cleavages. A dry brush, always.
Tools for this stone
Apophyllite: only the tools this stone actually concerns, each carrying the value read on this page.