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Variety · Sulfate · of Gypsum

Selenite

CaSO₄·2H₂O

The moonstone of the evaporites — gypsum's clear, fiber-optic variety.

Hardness2
Crystal systemMonoclinic
LusterVitreous
Sp. gravity2.3–2.33
ClassSulfate
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Selenite specimen

Selenite — Photo: Bryan Barnes · CC BY-SA 4.0

What is Selenite?

Selenite is the colorless, transparent variety of gypsum — CaSO₄·2H₂O — named for Selene, the Greek moon goddess, after its moonlit glow. The same mineral appears as silky satin spar, bladed desert roses, and the largest crystals ever found: Naica's Cave of Crystals holds beams of selenite up to 12 meters long.

It is one of the softest minerals — a fingernail scratches it — and cleaves into thin, flexible-looking sheets that ancient peoples sometimes used for window glass.

In one sentence: Scratches with a fingernail, splits into moonlit sheets — the softest of the common display minerals.

Physical Properties

The full data table is in the panel alongside. Here’s what those numbers actually mean.

Hardness. H 2 — the Mohs anchor. Fingernail hardness is the field test.

Twinning. Swallow-tail and fishtail twins are classic.

Giant growth. Naica crystals grew at a steady 58 °C for perhaps 500,000 years.

What It’s Used For

LapidaryCarved into lamps, spheres, and 'wands' — soft but luminous.
EducationThe standard mineral for demonstrating hardness and cleavage.

How to Identify It

H 2 — a fingernail scratches itPerfect one-direction cleavage into thin sheetsColorless transparent crystals, often twinnedMuch softer than calcite or quartz
Looks like…But differs because…Quick test
CalciteHarder (3), fizzes in acid, rhombohedral cleavageHardness & acid
QuartzFar harder (7), no cleavageHardness

Where It’s Found

Notable localities for fine specimens:

Naica Cave of Crystals, Mexico Great Salt Plains, United States Paris Basin, FranceOpen the locality map →