Native gold is the elemental form of the metal found naturally in the Earth’s crust, uncombined with other elements in a chemical compound. It exists as pure metal or as a natural alloy, rather than as a mineral ore that requires chemical separation. Gold is one of the few metals that occurs in this free, unoxidized state due to its chemical stability. This readily available native form was the source of the world’s first gold rushes and the gold used by early civilizations.
Defining the Natural State
Native gold is chemically designated as Au. While often described as pure, it is technically a solid solution or natural alloy. The most common natural impurity is silver, which forms a continuous alloy series. When the silver content exceeds about 20%, the alloy is known as electrum. Other elements like copper, iron, palladium, and mercury can also be present, influencing the gold’s color and physical properties.
The purity of native gold is quantified using the fineness system, which expresses the gold content in parts per thousand. A fineness of 850 means the material is 85% gold by weight. Pure gold has a fineness of 1,000, but native gold typically ranges between 650 and 999 fine. Placer gold often has a higher fineness due to the leaching of less stable impurities.
The metal’s extreme density is a primary physical trait, with pure gold having a specific gravity of 19.3. This makes it one of the heaviest naturally occurring elements. This high density causes gold to settle quickly in water, a property exploited in recovery methods like panning. Native gold is also exceptionally soft, registering only 2.5 to 3 on the Mohs hardness scale.
This softness translates to high malleability and ductility, meaning the gold can be easily beaten into thin sheets or drawn into fine wires without fracturing.
Native gold’s chemical inertness classifies it as a noble metal that does not react with oxygen or water. This resistance to corrosion means native gold does not tarnish or rust. It remains bright and metallic even after millions of years exposed to the environment.
Geological Occurrence and Forms
Native gold is found in two primary geological settings: lode deposits and placer deposits. Lode deposits represent the original source of the gold, typically disseminated within hard rock formations, most often quartz veins. This gold was deposited from hot, mineral-rich hydrothermal fluids circulating deep within the Earth’s crust, which precipitated the metal as the fluids cooled.
In lode deposits, gold often occurs as microscopic particles locked within sulfide minerals like pyrite, or as thin films and wires in fractures. To liberate the gold from lode deposits, the rock must be crushed and chemically processed, a method known as hard rock mining. Lode deposits are the ultimate source for all other forms of native gold.
Placer deposits are formed when lode deposits are exposed to weathering and erosion, breaking down the host rock over geologic time. Flowing water then transports the liberated gold particles, concentrating them in loose sediments in riverbeds, stream gravels, or alluvial fans. The dense nature of gold causes it to settle out of the water current, often accumulating in pockets or behind obstructions.
The physical forms of native gold vary significantly depending on their origin. In lode deposits, it can appear as branching crystalline structures, thin sheets, or delicate wires. Placer gold, due to mechanical tumbling during transport, is typically found as rounded nuggets, flattened flakes, or fine dust. Nuggets are the largest form of placer gold, representing pieces resistant to further breakdown and concentration.
Distinguishing Native Gold from Other Minerals
Native gold is frequently mistaken for other yellow, metallic-looking minerals, most notably pyrite (“fool’s gold”) and certain types of mica. The most reliable field test is the streak test, which involves rubbing the mineral across an unglazed porcelain plate. Real gold leaves a characteristic golden-yellow streak, whereas pyrite leaves a greenish-black streak, and mica leaves a white or brownish streak.
Another simple distinction is based on hardness and malleability. Native gold is so soft that it can be easily scratched or indented with a copper coin or a knife blade, and a flake will simply flatten when pressed. Pyrite is much harder; it will not be scratched and will shatter or crumble when force is applied, demonstrating its brittle nature.
The difference in specific gravity provides a final field test, which is the principle behind gold panning. Native gold is approximately four times denser than pyrite. When swirled in a pan of water, the high density of gold ensures it remains at the bottom of the pan, while the lighter pyrite and mica are quickly washed away with the surrounding sediment.