The Collaborative International Dictionary of English v.0.48:

Cerargyrite \Ce*rar"gy*rite\ (s[-e]*r[aum]r"j[y^]*r[imac]t), n.
   [Gr. ke`ras horn + 'a`rgyros silver.] (Min.)
   Native silver chloride, a mineral of a white to pale yellow
   or gray color, darkening on exposure to the light. It may be
   cut by a knife, like lead or horn (hence called horn
   silver).
   [1913 Webster]