Gypsum is an ordinary stone with a niche use for dwarven healthcare - once mined, it can be processed at a kiln to make gypsum plaster powder. A fort with no gypsum can use splints as an alternative.
Gypsum can also contain small clusters of selenite, satinspar, and alabaster, which can also be made into plaster powder, as well as anhydrite, which cannot.
Gypsum crystals (dirty yellow) on another mineral
Though most are not this clear, gypsum can be found as a transparent crystal
this is common gypsum, 3 vars below, any gypsum can be heated to and then mixed with water to make plaster
[SOLID_DENSITY:2320] Gypsum density was rather high. My sources all give ~2300.