The Collaborative International Dictionary of English v.0.48:

Pyrite \Pyr"ite\, n.; pl. Pyrites. [Cf. F. pyrite. See
   Pyrites.] (Min.)
   A common mineral of a pale brass-yellow color and brilliant
   metallic luster, crystallizing in the isometric system; iron
   pyrites; iron disulphide.
   [1913 Webster]

         Hence sable coal his massy couch extends,
         And stars of gold the sparkling pyrite blends. --E.
                                                  Darwin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pyrite
    n 1: a common mineral (iron disulfide) that has a pale yellow
         color [syn: pyrite, iron pyrite, fool's gold]