Native gold. Kalimantan, Indonesia
This is a gold concentrate from a river placer. The variation in colour is related to the fineness of the gold. Darker, more yellow, gold is close to 980 fine and forms rims around whiter, more argentian cores. The cores of the grains are themselves heterogeneous (bottom right) with a yellow core within a whiter margin (left of grain); this is difficult to see, but both are surrounded by a spongy, deep yellow, fine gold rim. One grain (top left) shows a relict core within a non-porous fine gold rim. Dark grey area is silicate (bottom left), black areas are erupted air bubbles within the resin. The green-grey colour of the resin is due to the incomplete removal of carbon-coating.
Grain mount, plane polarized light, x 80, air