What Is Selenite? A Stone That Dissolves in Water

Selenite is a crystalline form of gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O). It's genuinely soft — about 2 on the Mohs hardness scale, meaning it can often be scratched with a fingernail — and water-soluble, meaning prolonged water contact can dull its surface or, over enough time, dissolve it.

Gypsum's Different Forms

Gypsum occurs in several distinct forms depending on how it crystallizes: selenite refers specifically to clear, flat, plate-like or blade-like crystals; satin spar is a fibrous, silky-looking form of the same mineral; alabaster is a fine-grained, massive form traditionally carved into sculpture. All three are chemically identical gypsum, differing only in crystal habit and texture.

Desert Roses

In arid environments, gypsum (and sometimes the related mineral barite) can crystallize into distinctive rosette clusters resembling flower petals, formed as evaporating groundwater deposits the mineral around grains of sand — these "desert roses" are a genuinely different growth habit from the clear blade-like selenite typically used in jewelry and home décor.

The Naica Cave Crystals

Mexico's Naica Mine contains what are, by a wide margin, the largest natural gypsum crystals ever documented — some exceeding 10 meters in length. They formed under an unusual combination of conditions: a cavern flooded with mineral-saturated water held at a remarkably stable temperature near 58°C (136°F) for hundreds of thousands of years, allowing crystals to grow slowly and continuously to an extraordinary scale rarely seen anywhere else on Earth.

Its Name

Selenite's name comes from the Greek word for "moon" (selēnē), a reference to the pearly, gently glowing sheen visible along its cleavage planes.

Quick Facts

Mineral family: gypsum (calcium sulfate) — Hardness: 2 on the Mohs scale (very soft) — Water-soluble — Related forms: satin spar, alabaster — Famous large-crystal locality: Naica Mine, Mexico.