From 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]From WordNet (r) 3.0 (2006) :
cassiterite n 1: a hard heavy dark mineral that is the chief source of tin