Gold nuggets are a naturally occurring piece of native gold. Watercourses often concentrate and grow the nuggets. Nuggets are recovered by placer mining, but they are also found in residual deposits where the gold-bearing veins or lodes are weathered. Nuggets are also found in the tailings piles of previous mining operations, especially those left by gold mining dredges.
Many nuggets are formed by cold welding of smaller particles and fragments in streams and rivers. Gold’s softness makes it prone to welding under impact or hammering loads, such as those produced at waterfalls and in rapids. There is no oxide surface layer to inhibit adhesion between gold flakes. So when pieces of gold hit one another under pressure, they may join into a bigger piece. Some nuggets may have been present in massive form in the original vein before erosion, often showing signs of abrasive polishing by stream action. (Other precious metals such as platinum form nuggets in the same way.) The exceptional ductility and malleability of gold means brittle fracture is impossible.
Klondike nuggets typically range in purity from 79% to 89%. Gold nuggets in Australia often are 23K or slightly higher, while Alaskan nuggets are usually at the lower end of the spectrum. Purity can be roughly assessed by the nugget color, the richer and deeper the orange-yellow the higher the gold content. Nuggets are also referred to by their fineness, for example “865 fine” means the nugget is 865 parts per thousand in gold. The common impurities are silver and copper. Nuggets high in silver content constitute the alloy electrum.