Native gold and osmium-iridium alloy. Kalimantan, Indonesia
All three native gold grains have gold enrichment rims about a high fineness core (bottom left), and a low fineness core (light yellow, higher reflectance, right); the latter rim is spongy. An angular grain of osmium-iridium alloy (white, centre left), iridosmine, has an incomplete gold rim which either has been mechanically applied onto the alloy or has precipitated on it. At higher magnification a radiating habit (right-hand margin of alloy) suggests precipitation.
Grain mount, plane polarized light, x 80, air