Mineral · Sulfates
Gypsum
Ca(SO4)·2H2O
Gypsum is hydrated calcium sulfate; selenite is its clear crystal form (the giant crystals of Naica, Mexico), satin spar the fibrous form sold as 'selenite' wands, alabaster the massive form for carving and desert roses the sand-included rosettes of the Sahara and Oklahoma. It can be scratched with a fingernail (hardness 2) and dissolves slowly in water. Utah's Cave of the Swords and the swallowtail twins of Red River Floodway are classics.
| Formula | Ca(SO4)·2H2O |
|---|---|
| IMA status | grandfathered |
| Crystal system | Monoclinic |
| Mohs hardness | 2 |
| Group | Sulfates |
| Type locality | None (known since antiquity) |
| Streak | White |
| Specific gravity | 2.32 |
| Reference list | IMA List of Minerals (RRUFF) — verify formula and status against the current list |
Fakes and treatments
Satin spar wands and towers are sold as 'selenite crystals'; fine but mislabelled. Some Moroccan desert roses are cemented or trimmed.
Elsewhere
Cite this article
Stratum Study. "Gypsum" (Mineral). Stratum Study pack 2026.09, as of 2026-09-19. https://stratum.black/study/minerals/gypsum. Accessed 2026-09-20.
@misc{stratum-gypsum,
title = {Gypsum},
author = {{Stratum Study}},
howpublished = {\url{https://stratum.black/study/minerals/gypsum}},
year = {2026},
note = {Mineral article, Stratum Study pack 2026.09, as of 2026-09-19; sources: stratum, wikidata, commons. Accessed 2026-09-20.}
} Citations carry the pack version and the article's as-of date so they resolve to the same text later. The facts themselves belong to the sources listed below; cite those for the science.