Gem library
Selenite
Translucent gypsum, soft as a fingernail
Selenite is the crystallised, translucent variety of gypsum, one of the softest minerals there is: a fingernail scratches it. It also produced the largest crystals ever found, in the Naica cave in Mexico.
- Hardness (Mohs)
- 2
- Density
- 2.30 – 2.33
- Crystal system
- Monoclinic
- Refractive index
- 1.519 – 1.530
- Cleavage
- Perfect on {010}
Mineralogical identity
- Family
- Sulphates
- Species
- Gypsum
- Variety
- Selenite, satin spar, gypsum alabaster, desert rose
- Chemical formula
- CaSO₄·2H₂O
- Crystal system
- Monoclinic
- Classification
- Collector's mineral
- Rarity
- Very common
- Indicative price
- €0.05 – 2/ct
Etymology
From the Greek selene, the moon, for its pearly milky sheen — hence also «moonstone» in some ancient authors, a source of lasting confusion.
Physical & optical properties
- Hardness (Mohs)
- 2
- Density
- 2.30 – 2.33
- Refractive index
- 1.519 – 1.530
- Birefringence
- 0.010
- Cleavage
- Perfect on {010} — splits into sheets
- Fracture
- Fibrous to splintery
- Lustre
- Vitreous to pearly, silky if fibrous
- Diaphaneity
- Transparent to translucent
- UV fluorescence
- Weak
- Pleochroism
- None
- Colours
- white, colourless, pearl grey
Formation, geology & history
The giants of Naica
In 2000, miners at Naica in Mexican Chihuahua broke into a cavity 300 metres down and found a forest of selenite crystals more than 12 metres long weighing 55 tonnes: the largest natural crystals ever observed. Their growth, extraordinarily slow, spanned roughly 500,000 years in sulphate-saturated water held at 54 °C by the magma below. The cave is a thermal hell — 58 °C and 100 % humidity — where exploration lasts only a few dozen minutes with refrigerated gear. Since pumping stopped, the cavity has reflooded, and the crystals continue slowly growing underwater.
A soft, soluble mineral
Gypsum is a hydrated sulphate: it contains two water molecules per unit, which it loses from 100 °C to become plaster. That dehydration is reversible, which is exactly what makes plaster useful: mixed with water, it takes its molecules back and recrystallises into a solid mass. Selenite is therefore the ancestor of plaster of Paris, quarried for centuries from Montmartre and the hills of the Paris basin. Its hardness of 2 is proverbial: it is scratched by a fingernail, cut with a knife, slowly dissolved in water. It can in no way serve as a worn gem.
Varieties and forms
Selenite proper forms translucent blades and prisms, sometimes as spearheads or swallowtail twins. Satin spar, fibrous, shows a silky chatoyancy and is cut into cabochons or turned objects — the familiar selenite towers and wands of the trade belong here. Gypsum alabaster, massive and fine-grained, has been a sculpture material since Mesopotamian antiquity. The desert rose, finally, is a rosette of gypsum crystals formed in Saharan desert sands, where it traps quartz grains that give it its ochre colour.
How to recognise it
- Scratched by a fingernailHardness 2: a fingernail leaves a visible mark. It is the quickest test in the mineral kingdom, and it never lies.
- Splits into sheetsPerfect cleavage lets you lift thin, flexible blades with a knife tip.
- Lightness and warm touchDensity 2.3: it feels light, and its touch is softer and less cold than quartz or calcite.
Imitations, treatments & confusions
- Sold as moonstoneAn old confusion sustained by the etymology. Selenite is scratched by a fingernail, moonstone scratches glass: nothing in common.
- Resin and cast plasterCheap selenite towers and lamps are sometimes moulded plaster. No sheeted cleavage and no oriented fibrous structure.
- Confusion with calcareous alabasterThe true alabaster of sculptors is gypsum; calcareous alabaster is calcite. Vinegar settles it: calcite fizzes, gypsum does not.
Deposits & origins
Mexico (Naica, Chihuahua — the Cueva de los Cristales with its 12-metre crystals), Morocco (desert roses), Spain, United States (Utah, Oklahoma), Italy (Sicily), France (Paris basin, Montmartre gypsum), Australia
Care & precautions
- Never water in any form: gypsum is soluble and a simple immersion clouds and disintegrates it.
- Dust with a soft dry brush, or a dry microfibre cloth.
- Extremely fragile: hardness 2, perfect cleavage. Handle with both hands, store in cotton.
- Never heat it: above 100 °C it dehydrates into plaster and loses its transparency permanently.
Traditions & crystal healing
- Chakras
- Crown
- Zodiac signs
- Taurus · Cancer
- Element
- Water · Air
Tradition makes it a stone of cleansing and clarity, often used as a wand or tower placed in a room. It is credited with mental calm and a «reset» of a place's atmosphere. A useful irony: the tradition that recommends cleansing stones in water must on no account be applied to this one.
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
Can selenite go in water?
Absolutely not. Gypsum is water-soluble: even a brief immersion dulls the surface, and prolonged soaking disintegrates the piece. Clean it dry, with a brush.
Selenite or moonstone?
No relation, despite the name. Selenite is a soft gypsum (hardness 2, scratched by a fingernail); moonstone is a feldspar (hardness 6, scratches glass) with a mobile bluish veil.
Why is my selenite tower going dull?
Handling and humidity: gypsum absorbs fingerprints and ambient moisture matts its surface. A dry cloth revives the look slightly, but the matting is partly irreversible.