text: Gold occurs principally as a native metal, i.e., gold itself. Sometimes it is alloyed to a greater or lesser extent with silver, which is called electrum. Native gold can occur as sizeable nuggets, as fine grains or flakes in alluvial deposits, or as grains or microscopic particles (known as colour) embedded in rock minerals.  Other forms of gold are the minerals calaverite (AuTe), aurostibnite (AuSb), and maldonite (AuBi).  These latter three, although rarer that native gold, can be slow to react with cyanide and thus difficult to process. Still other gold-containing ores include various tellurides (sylvanite, nagyagite, petzite, and krennerite).
  Certain contaminants in ores can interfere with the extractability of gold by cyanide.  These interfering agents are called "preg-robbing ores".  For example, gold can bind tightly to carbon, resisting normal cyanide extraction.  Gold cyanides bind also to some clays.