Native gold and zircon. Kalimantan, Indonesia
This is a gold concentrate from a river placer. Gold grains show variation in their core colour and hence composition. Fine gold cores are darker yellow (right centre) than those grains with higher reflectance cores (centre and top left). High fineness rims are either well polished (top left) or spongy (centre). The contact between core gold and gold rim is sharp (less than 2śm). Pounded zircon grains (grey, top right) retain some of their original morphology. Rounded black areas are erupted air bubbles within the resin. Incomplete removal of carbon-coating along scratches in the resin is visible as blue-grey areas.
Grain mount, plane polarized light, x 80, air