The chemical composition of supergene gold nuggets can reveal the characteristics of the primary ore. Nuggets high in silver content constitute the alloy electrum. The common impurities are silver and copper. Nuggets are also referred to by their fineness, for example "865 fine" means the nugget is 865 parts per thousand in gold by mass. Purity can be roughly assessed by the nugget color, the richer and deeper the orange-yellow the higher the gold content. Gold nuggets in Australia often are 23K or slightly higher, while Alaskan nuggets are usually at the lower end of the spectrum. Nuggets are usually 20K to 22K purity (83% to 92% by mass). A later study of native gold from Arizona, US, based on lead isotopes indicates that a significant part of the mass in alluvial gold nuggets in this area formed within the placer environment. Other precious metals such as platinum form nuggets in the same way. A 2007 study on Australian nuggets ruled out speculative theories of supergene formation via in-situ precipitation, cold welding of smaller particles, or bacterial concentration, since crystal structures of all of the nuggets examined proved they were originally formed at high temperature deep underground (i.e., they were of hypogene origin). They often show signs of abrasive polishing by stream action, and sometimes still contain inclusions of quartz or other lode matrix material. Nuggets are gold fragments weathered out of an original lode. A large gold nugget from Nevada County, California
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