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Chalcopyrite vs Gold: Streak, Tarnish and Deformation Tests

Chalcopyrite is a widespread copper-iron sulfide whose bright brass-yellow metallic surface can be mistaken for native gold. Fresh colour alone is unreliable, especially in small grains, photographs, or mixed sulfide material. Chalcopyrite commonly develops blue, purple, or green iridescent tarnish, leaves a greenish-black to black streak, and breaks or crumbles under stress. Native gold is softer, vastly denser, and malleable: a suitable expendable grain tends to dent, flatten, bend, or smear instead of shattering. Use several independent clues and an assay for any consequential decision. On an expendable loose grain, gentle pressure and streak are usually the clearest field separators. Chalcopyrite is brittle and leaves a greenish-black to black streak; gold deforms and leaves a yellow streak. Iridescent tarnish supports chalcopyrite, while comparable-volume heft strongly favours gold, but neither colour nor one field test confirms an occurrence. Photograph the unaltered material in diffuse light with a scale and record the exact host rock or sediment context. Examine several grains rather than only the brightest patch. Use a hand lens to compare fresh brass colour, iridescent tarnish, grain boundaries, and whether the material is a coating, a discrete grain, or part of a mixed sulfide aggregate. If a loose, expendable grain is available and ownership and safety permit, press it gently with a steel point on a clean hard surface. Record whether it fractures or instead dents and flattens. Use an unglazed porcelain streak plate only on expendable material. A greenish-black to black streak supports chalcopyrite; a yellow streak supports gold. Clean the plate between grains to avoid mixed results. Compare hardness carefully on a fresh surface. Chalcopyrite is typically harder than gold but softer than pyrite; attached quartz can create a misleading scratch result. Compare heft only between similar volumes and treat it as supporting evidence. Tiny grains, porous material, host rock, and mixed mineral aggregates prevent a reliable hand-density judgment. When the result could affect money, land, ownership, reporting, or exploration decisions, preserve representative material and field notes and use a qualified laboratory assay suited to the sample. Do not grind, drill, heat, roast, acid-test, or taste unknown sulfide material. Avoid creating dust and follow approp