Native gold and zircon. Kalimantan, Indonesia
This is a gold concentrate from a river placer. A gold grain with a silver-rich core (pale yellow, centre) is enclosed within a spongy gold enrichment rim (yellow). Other gold grains are finer and have very thin or no gold enrichment rims. Zircon grains (grey, bottom left) show faint internal reflections. The rounded black area is an air bubble within the carbon-coated resin (green-grey).
Grain mount, plane polarized light, x 80, air