Real gold does not oxidize or tarnish under normal conditions. Oxidation is a chemical reaction where a substance loses electrons, often by combining with oxygen, like the process that causes iron to rust. Pure gold (24-karat gold) possesses a unique chemical stability that prevents this reaction in air or water. The dark discoloration commonly seen on gold jewelry is not the gold reacting, but rather the other metals mixed with it.
Gold’s Chemical Inertness and the Noble Metal Status
Pure gold is classified as a noble metal, a group that includes elements like platinum, which are highly resistant to chemical attack and corrosion. This exceptional stability is rooted in the metal’s atomic structure. Gold has a very stable electron configuration, meaning its outer electrons are not easily available to form chemical bonds with other elements.
The reluctance of gold atoms to share or lose electrons prevents them from readily reacting with common atmospheric elements like oxygen and water. This chemical inertness means gold does not form the oxides that cause rust on iron or the sulfides that cause tarnish on silver. Gold sits near the bottom of the reactivity series, making it one of the least reactive elements. Even strong acids, with the exception of aqua regia (a mixture of nitric and hydrochloric acids), typically have little effect on pure gold.
This inherent lack of reactivity is why 24-karat gold, which is 99.9% pure, does not lose its luster over time. This stability is the primary reason gold has been valued as both a currency and a material for enduring jewelry for thousands of years.
The Real Cause of Tarnishing in Jewelry
When gold jewelry darkens or discolors, it indicates the piece is not made of pure gold. Jewelers use gold alloys—mixtures of gold with less expensive and more durable metals—to increase strength, change color, and reduce cost, since pure gold is too soft for everyday wear. The tarnishing process is a chemical reaction involving these other metals, primarily copper and silver.
The most common form of discoloration on gold alloys is sulfidation, not true oxidation. This occurs when the copper and silver in the alloy react with sulfur compounds present in the air, moisture, or certain chemicals. Trace amounts of hydrogen sulfide gas are naturally present in the atmosphere, and this gas reacts with the alloy metals to form a dark film of metal sulfide on the surface.
Contact with common household substances can accelerate this reaction. Chlorine in swimming pools, salts and acids in human perspiration, and chemicals in cosmetics or cleaning products all act as catalysts. Rose gold, for example, contains a higher percentage of copper, making it more prone to tarnishing than standard yellow gold. The tarnish film is superficial and can usually be polished away.
Interpreting Carat Ratings
The likelihood of a gold item tarnishing is directly proportional to its carat rating, a system that measures the purity of the gold content out of 24 parts. Pure gold is defined as 24-karat (24K), meaning all 24 parts are gold. Since pure gold does not tarnish, a 24K piece will remain completely untarnished.
Lower carat ratings indicate a higher proportion of reactive alloy metals mixed in with the gold. For instance, 18K gold contains 18 parts gold and 6 parts alloy, equating to 75% purity, and is highly resistant to tarnishing. A 14K piece contains 14 parts gold and 10 parts alloy, making it 58.3% gold, and is noticeably more susceptible to discoloration. Jewelry with the lowest common ratings, such as 10K (41.7% gold), will tarnish much more frequently because the majority of the metal content is composed of reactive metals like copper, silver, and zinc. This direct relationship between the carat number and the percentage of noble metal determines the item’s overall resistance to environmental factors. Choosing a higher carat gold is the most effective way to ensure long-term, tarnish-free wear.