File:Gold nugget (Australia) 4 (16848647509).jpg

Gold nugget from Australia. (public display, Field Museum of Natural History, Chicago, Illinois, USA). The Australian rock shown above is a large, nearly four pound mass of gold. The irregularly-distributed, smoothly sculpted surfaces indicate that this is likely a fluvial gold cobble - in other words, it appears to be from a placer deposit.

Gold is a metal. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state as minerals. A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. As of the year 2018, there are around 4500 named and described terrestrial minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates. Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known. Of these, 94 occur naturally on Earth (hydrogen to plutonium).

Gold is a native element in nature. To find a native element, it must be relatively non-reactive and there must be some concentration process. Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals. Gold is very rare in crustal rocks - it averages about 5 ppb (parts per billion). Where gold has been concentrated, it occurs as wires, dendritic crystals, twisted sheets, octahedral crystals, and variably-shaped nuggets. It most commonly occurs in hydrothermal quartz veins, disseminated in some contact- & hydrothermal- metamorphic rocks, and in placer deposits. Placers are concentrations of heavy minerals in stream gravels or in cracks on bedrock-floored streams. Gold has a high specific gravity (about 19), so it easily accumulates in placer deposits. Its high density allows prospectors to readily collect placer gold by panning.