The origin of gold nuggets is a matter of debate. Many gold nuggets are formed as clusters of gold crystals from very hot water in cracks and fissures in hard-rocks, often with quartz. Later, weathering released the gold nuggets that end up in a stream due to gravity. But many gold nuggets are found in areas where there seems to be no hard-rock gold other than sometimes lots of microscopic gold.
Five rival theories, and they might all be right:
1- Gold nuggets form because gold is malleable, bendable and sticky. During transport small gold particles may stick together or indeed be cold-welded together by pressure and shearing in movement of gravels down a slope or in a stream bed. Shearing and hammering creates a small nugget and then they snowball together by squeezing, hammering and shearing in moving gravels. This sounds impossible but there is a lot of anecdotal evidence for it. Gold is soft, and behaves like extremely heavy chewing gum that gathers together and sticks together. It is interesting that often gold particles get bigger as nuggets for a few km downstream of the gold source, whereas platinum just seems to get smaller and smaller and platinum nuggets are correspondingly rare – platinum isn’t sticky!
2- Gold nuggets form because groundwater becomes rich in dissolved gold which sooner or later precipitates in soils and gravels as nuggets. The theory is that the gold precipitates on gold, and so the mass of crystals grows and grows. Now it seems that as well as routine precipitation of gold, the main cause is the role of special bacteria in causing precipitation.
3- Gold nuggets form from hard-rock gold that is now completely worn away, and so the gold prospector cannot find any evidence of hard-rock gold. It was there, but the evidence has now GONE!
4- Gold nuggets in modern placers come from old placers being eroded. The older placers, known as paleoplacers might be just a few thousand years old such as river terraces, or be millions of years old.
5- Gold nuggets weathered out not from hard-rock gold in a zone of weak shattered rock that the stream eroded. Often hard-rock gold is missed, being hidden under the placer!
Nuggets are gold fragments weathered out of an original lode. They often show signs of abrasive polishing by stream action, and sometimes still contain inclusions of quartz or other lode matrix material.
Other precious metals such as platinum form nuggets in the same way. Nuggets are usually 20.5K to 22K purity (83% to 92% by mass). 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.
Most nuggets are between 85 percent and 95 percent pure gold, but the remainder can be one of several kinds of minerals. Nuggets in laterite can be either reddish or black; nuggets in quartz appear cloaked with white.
A specimen gold nugget is a matrix of gold and other rock, usually quartz or ironstone (in Australia). If the gold to rock ratio is high, and the shape shows off a lot of the gold at the surface, your nugget can hold a higher value. The largest specimen gold nugget in the world to this date is the Holterman Nugget found in Australia at Hill End, NSW in 1872 weighing in at 285 kg.