Is gypsum a mineral?
Gypsum is a mineral species, CaSO4·2H2O, hardness 1.5 to 2. Calcined, it becomes plaster of Paris: the formula of the mineral bassanite, but manufactured.
Yes. Gypsum is a mineral: hydrated calcium sulphate, CaSO4·2H2O, monoclinic, hardness 1.5 to 2, with its own row on the September 2026 IMA list. Plaster of Paris is not: it is calcium sulfate hemihydrate, manufactured by calcining, and keeps a quarter of the water. Worked, 1,000 g of gypsum gives 843 g of hemihydrate.
In short
- Gypsum passes the definition of a mineral. It is a naturally occurring, homogeneous, crystalline solid of definite composition. The Handbook of Mineralogy sheet gives CaSO4·2H2O, monoclinic, hardness 1.5 to 2, density 2.317; the IMA-CNMNC list writes Ca(SO4)(H2O)2, status G, grandfathered.
- The water-count rule: calcium sulphate with 2 waters is gypsum, with half a water bassanite, with none anhydrite. A script over the September 2026 IMA list finds exactly those 3 rows whose formula is calcium sulphate alone, with or without water. All three are minerals when natural.
- Plaster of Paris has bassanite's formula but is made, not found. PubChem files plaster of Paris as calcium sulfate hemihydrate, with dried gypsum among its synonyms, and the USGS reports calcined gypsum as a manufactured product. Made in a kiln, it fails the definition's naturally occurring test; bassanite, the same composition formed in nature, passes.
- The arithmetic, worked on standard atomic weights: gypsum, 172.16, is 20.93% water; the hemihydrate, 145.14, is 6.21% water. Going from one to the other loses 15.70% of the mass, so 1,000 g of gypsum gives 843 g and releases 157 g of water. Two waters become half a water: a quarter is kept.
- Synthetic gypsum is not a mineral specimen either. The USGS says synthetic gypsum from industrial processes, including flue gas desulfurization of smokestack emissions, made up about 34% of the US domestic gypsum supply in 2024. Drywall, plasterboard and garden-gypsum completions are building and farming questions, outside this page.
| Material | Formula | Water by mass, worked | Status | Hardness (Handbook) | Density (Handbook) | A mineral? |
|---|---|---|---|---|---|---|
| Gypsum | Ca(SO4)(H2O)2 (IMA); CaSO4·2H2O (Handbook) | 20.93% | IMA row, G, no year | 1.5 to 2 | 2.317 | Yes |
| Bassanite | Ca(SO4)·0.5H2O (IMA); 2CaSO4·H2O (Handbook) | 6.21% | IMA row, G, 1910, Italy | Not determined | 2.69 to 2.76 | Yes |
| Anhydrite | Ca(SO4) (IMA); CaSO4 (Handbook) | 0% | IMA row, G, 1804, Austria | 3 to 3.5 | 2.98 | Yes |
| Plaster of Paris | Calcium sulfate hemihydrate (PubChem) | 6.21% | No IMA row; calcined, manufactured | Not a sheet value | Not a sheet value | No |
| Synthetic gypsum | Not stated in the USGS source | Not worked | No IMA row; industrial by-product (USGS) | Not a sheet value | Not a sheet value | No |
| Selenite, satin spar, alabaster | Gypsum | 20.93% | Variety names; no IMA rows | 1.5 to 2 | 2.317 | Yes, as gypsum |
Is gypsum a mineral? The definition applied
Gypsum is a mineral because natural gypsum is a homogeneous crystalline solid of definite composition, and it has an IMA row. The Handbook of Mineralogy gypsum sheet gives CaSO4·2H2O, monoclinic, prismatic to tabular crystals to 17 m, hardness 1.5 to 2, density 2.317, with a flexible, inelastic tenacity. The September 2026 IMA list writes the same compound Ca(SO4)(H2O)2, status G, with no year and the country unknown.
The water is part of the structure, not damp. The two waters sit in the formula, and they are 20.93% of the mass on standard atomic weights, the sheet's own ideal column. Lose them and the material is no longer gypsum. Where it forms, its twins and its giant crystals are on gypsum; the mineral-versus-crystal distinction is on crystal vs mineral.
Is gypsum the same as plaster of Paris? The water-count rule
The water-count rule: two waters, gypsum; half a water, bassanite; none, anhydrite. Method: the September 2026 IMA list as text, each data row parsed, and the formula matched to calcium sulphate alone with or without water; a raw line scan gave the same three rows. Bassanite is G, 1910, Italy, and anhydrite G, 1804, Austria.
Plaster of Paris sits on the half-water line, but it is made. The PubChem record for calcium sulfate hemihydrate is titled calcium sulfate, hemihydrate, and lists plaster of Paris, dried gypsum and calcined gypsum among its synonyms. The USGS reports calcined gypsum, from crude and synthetic gypsum, as a separate line of production. Worked: gypsum is 172.16 and the hemihydrate 145.14, so calcining to the hemihydrate loses 15.70% of the mass; 1,000 g gives 843 g, and 157 g of water goes. Removing all the water, to anhydrite's composition, loses 20.93%.
Bassanite is the natural hemihydrate. The Handbook of Mineralogy bassanite sheet gives 2CaSO4·H2O, white needlelike crystals to 0.1 mm, density 2.69 to 2.76, and an ideal water content of 6.21%, the same figure the arithmetic gives. It records bassanite as altered from gypsum at Vesuvius and in dry lake beds. The sheet's own cell and powder data are from synthetic material, which shows how close the two are; on the definition, origin is what separates them.
Is gypsum a rock? Mineral, rock and wallboard
Gypsum is the mineral; a bed of it is a rock. The Handbook sheet calls gypsum a common constituent of sedimentary rocks, particularly marine salt deposits, and also lists it in soils, in caves and as efflorescences in mines. The same word is used in industry for the mined rock: the USGS Mineral Commodity Summaries 2025, gypsum reports US crude gypsum production of about 22 million tons in 2024.
Synthetic gypsum is a by-product. The USGS describes it as generated by various industrial processes, including flue gas desulfurization of smokestack emissions, and estimates it at about 34% of the 2024 US domestic supply. It is a manufactured material, not a mineral specimen. Wallboard and plaster products are what most gypsum becomes; they are outside a collecting site's scope.
Variety names are gypsum. Selenite, satin spar and alabaster are names for forms of the one species, defined on the gypsum species page. Water and selenite are on is selenite water soluble and why is my selenite turning yellow.
Is gypsum toxic? What this page will and will not say
This page gives no health verdict on gypsum or plaster of Paris. One regulatory fact is relevant to the mineral question: the HSE EH40/2005 workplace exposure limits table lists gypsum, CAS 10101-41-4, and plaster of Paris, CAS 26499-65-0, as separate entries. UK regulation treats them as two substances, which matches the water-count rule.
For any health or workplace question, use HSE EH40 and COSHH 2002 or ask a clinician; none of this is advice. Specimen-handling questions in general are on toxic minerals in a collection. More definitions are in the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.