The Collaborative International Dictionary of English v.0.48:

Cassiterite \Cas*sit"er*ite\, n. [Gr. ? tin.] (Min.)
   Native tin dioxide; tin stone; a mineral occurring in
   tetragonal crystals of reddish brown color, and brilliant
   adamantine luster; also massive, sometimes in compact forms
   with concentric fibrous structure resembling wood (wood
   tin), also in rolled fragments or pebbly (Stream tin). It
   is the chief source of metallic tin. See Black tin, under
   Black.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cassiterite
    n 1: a hard heavy dark mineral that is the chief source of tin