Gypsum is a hydrous calcium sulfate mineral that is common in sedimentary deposits and as crack-filling vein material in near-surface soils and rocks.
Pronunciation: JIP•some (accent on capitalized syllable)
Color: varies from colorless to white or gray, but is easily colored by impurities to shades of yellow, red, brown...
Luster: vitreous (selenite gypsum) to silky (fibrous gypsum)
Diaphaneity: transparent to translucent; may appear opaque in hand specimen
Hardness: 2
Specific gravity: 2.32
Cleavage/fracture: 1 primary direction and a second direction that may be less well defined.
Other distinguishing properties: Gypsum is soft enough on cleavage faces to be scratched with a fingernail. It may be mistaken for calcite, which has 3 perfect directions of cleavage, is harder than gypsum, and effervesces in dilute hydrochloric acid. Gypsum might also be mistaken for fluorite (harder than gypsum, 4 directions of cleavage), quartz (much harder than gypsum, no cleavage), or halite (tastes salty, 3 directions of cleavage).
Klein, C., and Hurlbut, C.S., Jr., 1999, Manual of Mineralogy (after James D. Dana) [21st edition, revised]: New York, John Wiley & Sons, 682 p.
Los Angeles County Museum of Natural History, The Photo Atlas of Minerals: nhm.org/pam/
Return to the Minerals page.
