The Collaborative International Dictionary of English v.0.48:

Gypsum \Gyp"sum\ (j[i^]p"s[u^]m), n. [L. gypsum, Gr. gy`psos;
   cf. Ar. jibs plaster, mortar, Per. jabs[imac]n lime.] (Min.)
   A mineral consisting of the hydrous sulphate of lime
   (calcium). When calcined, it forms plaster of Paris.
   Selenite is a transparent, crystalline variety;
   alabaster, a fine, white, massive variety.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gypsum
    n 1: a common white or colorless mineral (hydrated calcium
         sulphate) used to make cements and plasters (especially
         plaster of Paris)

U.S. Gazetteer Places (2000):

Gypsum, CO -- U.S. town in Colorado
   Population (2000):    3654
   Housing Units (2000): 1210
   Land area (2000):     3.680365 sq. miles (9.532101 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    3.680365 sq. miles (9.532101 sq. km)
   FIPS code:            33695
   Located within:       Colorado (CO), FIPS 08
   Location:             39.644499 N, 106.940232 W
   ZIP Codes (1990):     81637
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Gypsum, CO
    Gypsum


U.S. Gazetteer Places (2000):

Gypsum, KS -- U.S. city in Kansas
   Population (2000):    414
   Housing Units (2000): 179
   Land area (2000):     0.432824 sq. miles (1.121009 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.432824 sq. miles (1.121009 sq. km)
   FIPS code:            29250
   Located within:       Kansas (KS), FIPS 20
   Location:             38.705118 N, 97.426876 W
   ZIP Codes (1990):     67448
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Gypsum, KS
    Gypsum