The question of whether real gold can change color is a common one, but the definitive answer is that the pure element itself does not turn silver or tarnish. Gold (Au) is chemically stable, and any visible discoloration is caused by other metals mixed with or placed on the gold. When a piece of jewelry appears to change color, it is a reaction involving non-gold components, surface treatments, or external contaminants. These changes can range from slight darkening to the appearance of a silver or white layer.
The Chemical Stability of Pure Gold
Pure gold is classified as a noble metal, recognized for its remarkable resistance to corrosion and oxidation. This chemical inertness is due to the tightly bound electrons in the gold atom’s structure, preventing them from easily reacting with other substances. Because of this stable configuration, elemental gold does not interact with oxygen in the air or water to form oxides, which is the process known as rusting or tarnishing.
The purest form of gold, known as 24-karat, contains 99.9% or more of the gold element and serves as the benchmark for this chemical stability. This concentration resists virtually all common acids and bases, remaining bright and yellow even when exposed to harsh environments. Only highly specialized and corrosive mixtures, such as aqua regia (a blend of nitric and hydrochloric acids), possess the chemical power to dissolve elemental gold. This inherent chemical resilience ensures that the gold component maintains its original color indefinitely.
The Reactions of Alloying Metals
While pure 24-karat gold is chemically non-reactive, it is too soft for most jewelry applications and must be mixed with other metals to create a durable alloy. Jewelry pieces marked with lower karats (18k, 14k, or 10k) contain significant percentages of these other metals, which are the source of discoloration. Common alloying metals include copper, which adds hardness and a reddish tint, and silver, which contributes to a lighter yellow or white color.
These non-gold metals react with everyday substances, leading to observed color changes. Copper reacts readily with sulfur compounds found in the air or on the skin to form dark copper sulfides, manifesting as a black or greenish tarnish. Similarly, silver in the alloy reacts with airborne sulfur to form silver sulfide, a black compound that darkens the metal. Corrosive agents are often present in human sweat, cosmetics, cleaning products, or moisture in the air.
The lower the karat, the higher the proportion of these reactive metals, making pieces like 10k gold more susceptible to tarnishing than 18k gold, which has a higher gold content. These reactions are surface-level and involve the creation of a thin film of chemical compounds, which changes the perceived color of the metal. This effect is often mistaken for the gold itself changing color, when it is actually the corrosion of the silver, copper, or zinc components.
Plating Failure and External Reactions
A common cause for a gold item appearing to change color is the failure of a surface treatment, particularly in white gold jewelry. White gold is naturally an off-white or yellowish-gray color because it is an alloy of gold and white metals like palladium or nickel. To achieve the bright, white finish customers expect, this alloy is routinely coated with a thin layer of rhodium, a silvery-white noble metal.
The rhodium plating is hard and tarnish-resistant, but it is not permanent; it wears down over time, especially on high-friction areas like the underside of a ring. As the rhodium layer wears away, the underlying, slightly yellowish or warm-gray white gold alloy is revealed, creating the illusion that the gold is turning yellow or dull. For frequently worn items, this requires professional re-plating every one to two years to restore the crisp, white surface.
In rare instances, gold can appear silver or white due to contact with elemental mercury, a process called amalgamation. Mercury binds with the gold surface, forming a silvery-white compound known as an amalgam. This temporary color change is a physical and chemical bonding process that coats the gold with the silver-colored mercury-gold mixture. This can occur through exposure to mercury-containing products, and the silvery appearance can be reversed by carefully heating the item to vaporize the mercury.