The Collaborative International Dictionary of English v.0.48:

Colcothar \Col"co*thar\ (k[o^]l"k[-o]*th[~e]r), n. [NL.
   colcothar vitrioli, fr. Ar. qolqo[.t]ar.] (Chem.)
   Polishing rouge; a reddish brown oxide of iron, used in
   polishing glass, and also as a pigment; -- called also
   crocus Martis.
   [1913 Webster]