pearceite

from The Century Dictionary.

• noun A sulpharsenite of silver, Ag₉AsS₆, analogous to the sulphantimonite polybasite, to which it is also related in crystallization: found in Montana and also in Colorado and Utah.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mineralogy A trigonal-hexagonal scalenohedral black mineral containing antimony, arsenic, copper, silver, and sulfur.