Anhydrite is a magma-safe stone of base value that is only found within various types of gypsum. In real life, anhydrite is essentially dehydrated gypsum, and on contact with water will immediately hydrate and soften; however in Dwarf Fortress, anhydrite floodgates are just as effective as floodgates made from granite. Oddly, unlike the gypsum in which it occurs, anhydrite cannot be used to make gypsum plaster.
gypsum without any water (plaster still has some)
[SOLID_DENSITY:2960] Significantly denser than the other gypsum-related minerals.