Native gold, platinum-iron alloy and zircon. Kalimantan, Indonesia

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This is a gold concentrate from a river placer. Gold grains have well polished gold enrichment rims (bottom left and right) around high fineness gold cores (slightly higher reflectance and lighter surface colour). A spongy gold enrichment rim has developed around a silver-rich gold core (top left). A rounded grain of platinum-iron alloy (centre) is white when compared with silver-rich cores within gold grains (top left). Zircon grains are dark grey and are subhedral (centre top). Rounded black areas are erupted air bubbles within carbon-coated resin (green-grey).

Grain mount, plane polarized light, x 80, air

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