What is "horn silver"

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).

n. (lb en mineral) A weathered form of chlorargyrite