Gold nuggets of various sizes have been found throughout the world. Historically, the nuggets are melted down and formed into new objects. The Welcome Stranger is the largest alluvial gold nugget ever found, which had a calculated refined weight of 97.14 kilograms (3,123 ozt). Three of the biggest nuggets come from the Brazilian Serra Pelada mine. Most of the largest nuggets were melted down into ingots and so only have historical records of their size and mass.

A gold nugget is a naturally occurring piece of native gold. Watercourses often concentrate nuggets and finer gold in placers. 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.

Nuggets are gold fragments weathered out of an original lode.^[1] They often show signs of abrasive polishing by stream action, and sometimes still contain inclusions of quartz or other lode matrix material. A 2007 study of Australian nuggets ruled out speculative theories of supergene formation via in-situ precipitation, cold welding of smaller particles, or bacterial concentration, since the crystal structures of all nuggets examined proved they were originally formed at high temperature deep underground (i.e., they were of hypogene origin).^[2][3]

Nuggets are usually 20.5 to 22 karat (k) purity, meaning they are 83% to 92% gold by mass. Gold nuggets in Australia are often 23 k or slightly higher, while Alaskan nuggets are usually at the lower end of the spectrum. Purity can be roughly assessed by 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 by mass. The common impurities are silver and copper. Nuggets high in silver content constitute the alloy electrum.^[4]

Two gold nuggets are claimed as the largest in the world: the Welcome Stranger and the Canaã nugget, the latter being the largest surviving natural nugget. Considered by most authorities to be the biggest gold nugget ever found, the Welcome Stranger was found at Moliagul, Victoria, Australia in 1869 by John Deason and Richard Oates. It had a gross weight of over 2,520 troy ounces (78 kg; 173 lb) and returned over 2,284 troy ounces (71.0 kg; 156.6 lb) net.^[5] The Welcome Stranger is sometimes confused with the similarly named Welcome Nugget, which was found in June 1858 at Bakery Hill, Ballarat, Australia by the Red Hill Mining Company. The Welcome Nugget weighed 2,218 troy ounces (69.0 kg; 152.1 lb). It was melted down in London in November 1859.^[6]

Large nuggets are still being found around the world. On 16 January 2013, a large gold nugget was found near the city of Ballarat in Victoria, Australia by an amateur gold prospector. The Y-shaped nugget weighed slightly more than 5 kilograms (11 lb), measured around 22 cm high by 15 cm wide, and has a market value slightly below 300,000 Australian dollars, though opinions have been expressed^{[by whom?]} that it could be sold for much more due to its rarity. The discovery has cast doubt on the common rumour that Victoria's goldfields were exhausted in the 19th century.^[7]

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