Selenite is the crystalline form of gypsum — hydrated calcium sulphate — and in mineralogy it means the clear, colourless, tabular or bladed crystals. Most of what is sold as "selenite" in wands, towers, lamps and bowls is satin spar, the fibrous, silky-white form of the same mineral, which is cheaper, more abundant and easy to cut. Both are gypsum; both are genuine; the naming is loose rather than fraudulent. What matters more to a buyer is that gypsum is among the softest and most water-sensitive materials sold as a "crystal".
What gypsum is
Calcium sulphate dihydrate. Properties: hardness 2 (a fingernail scratches it), SG 2.30–2.33, RI 1.52–1.53 (two readings, small birefringence), doubly refractive, perfect cleavage in one direction, and — critically — slightly soluble in water. It is inert or weakly fluorescent under UV.
Selenite proper: transparent, colourless, in flat plates or blades that cleave into thin sheets; sometimes "fishtail" twins; occasionally large crystals (the Naica cave crystals are selenite).
Satin spar: masses of fine parallel fibres with a silky, chatoyant white lustre — a band of light moves across a polished surface. This is the material of most wands and towers.
Desert rose: rosettes of gypsum crystals with included sand.
Alabaster: fine-grained massive gypsum, carved into ornaments.
What it is confused with
Calcite. Also soft (3), but harder than gypsum; higher SG (2.71); very strong double refraction (a line viewed through clear calcite is obviously doubled); effervesces in acid. Optical calcite is sold as "clear selenite" occasionally.
Ulexite ("TV rock"). A fibrous borate that transmits an image along its fibres; softer and rarer.
Quartz and glass. Far harder (7 and 5.5); a piece that cannot be scratched by a fingernail is not gypsum.
Plastic. Warm, light, no cleavage or fibre structure.
Because gypsum is so soft, a fingernail test on an inconspicuous edge is one of the few scratch tests that is nearly diagnostic — though the laboratory prefers SG, RI and the polariscope.
Care: the important part
Water dissolves gypsum, slowly. A selenite bowl left damp develops a rough, etched surface; a wand rinsed under a tap loses its polish; prolonged soaking destroys the piece. It should never be cleansed in water, salt water or by burial in salt. Humidity alone, over years, can dull it.
It scratches with anything. A fingernail, a coin, other crystals in the same box. Store separately, wrapped.
It cleaves. A dropped selenite blade splits into sheets; a satin spar tower breaks along its fibres.
Heat dehydrates gypsum to plaster.
How the laboratory identifies it
Hardness by inference, SG (2.32), RI on any flat cleavage face, polariscope (doubly refractive; satin spar shows the fibre structure), and appearance. Report states: natural gypsum (selenite or satin spar). Treatment is essentially never encountered; dyed satin spar in bright colours exists and is reported as dyed.
Frequently asked questions
Is satin spar "fake" selenite?
No. It is the same mineral in a different form. Sellers use "selenite" for both; a precise description would say satin spar for the fibrous white material.
Can I clean selenite in water?
No. It dissolves. Wipe with a dry or barely damp cloth and dry immediately.
Why is my selenite cloudy or rough?
Water exposure, humidity or handling. Etched surfaces cannot be restored without repolishing.
Is coloured selenite natural?
Peach and orange satin spar occur naturally from iron. Bright blue, green and pink are dyed.
Is selenite worth testing?
It is inexpensive and easily recognised. Testing is rarely needed except to identify an unfamiliar piece.