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chalcocite (copper glance) Mineral, Cu2S; sp. gr. 5.5–5.8; hardness 2.5–3.0; orthorhombic; dark lead-grey tarnishing to black; black streak; metallic lustre; prismatic and tabular crystals are rare, it is usually found massive, or as powdery coatings; cleavage prismatic, poor; can occur in hydrothermal veins in a primary state, but more usually found in the zones of supergene enrichment of copper-ore bodies. It is an important ore mineral for copper.