Partzite from Mona County in California
Partzite is a copper antimony oxy-hydroxide, near to Cu2Sb2(O,OH)7 in composition, formed by weathering of copper-antimony sulfosalts. It is generally bluish to olive green, and (as in this specimen) tarnishes black. This specimen is about 7 cm.
University of North Dakota Mineralogy Collection, #581.