Pure gold, in its elemental form, possesses a singular, distinct color. Because pure gold is too soft for durable jewelry, it is mixed with other metals to create alloys. These gold alloys are used in nearly all jewelry and give the metal its diverse color palette. Understanding the science behind gold’s natural color and how other elements alter its composition is key to distinguishing between the base metal and its engineered variations.
The Distinct Color of Pure 24K Gold
The color of pure, 24-karat gold is a deep, rich, metallic yellow, a hue rare among metals. Most pure metals, like silver and platinum, appear silvery-white because their electron structures reflect all wavelengths of visible light equally. Gold has a unique electron configuration influenced by relativistic effects, causing its outer electrons to move at high speeds. This movement reduces the energy required for the electrons to absorb light, specifically absorbing the higher-energy, bluer wavelengths. The remaining lower-energy light, composed of yellow and red wavelengths, is reflected back to the eye, giving pure gold its characteristic warm color.
How Alloying Creates White, Rose, and Green Gold
Alloying is the primary technique used to create the spectrum of colors seen in jewelry. The proportion and type of non-gold metal added fundamentally change the optical properties of the final material.
Rose Gold
Rose gold achieves its distinctive pinkish-red tone by being alloyed with a significant amount of copper. Copper is the only other pure metal besides gold that exhibits a non-silvery color. A typical 18K rose gold alloy contains 75% gold and approximately 25% copper, with small traces of silver sometimes included to lighten the shade. The greater the copper content, the more saturated the reddish hue becomes.
White Gold
White gold is created by mixing pure gold with white metals like palladium, nickel, or platinum, along with small amounts of zinc or copper. This combination masks the natural yellow color of the gold, resulting in a paler, grayish-yellow alloy. Nickel-based white gold alloys are widely used because they are stronger and less expensive. However, palladium-based alloys are preferred for their hypoallergenic properties and superior whiteness.
Green Gold
Green gold, also known historically as electrum, is achieved by alloying gold with silver. Silver, a white metal, dilutes the gold’s yellow tone while adding a subtle greenish tint. Green gold alloys typically contain a high percentage of silver, sometimes including copper and zinc to adjust the final shade. This color is rarely used for entire pieces of jewelry, often appearing as an accent color.
Karat Value and Color Intensity
The intensity and saturation of a gold item’s color are directly linked to its purity, measured by the karat system based on 24 parts. Lower karat values signify a higher proportion of non-gold alloy metals. This higher proportion either dilutes the natural yellow color or intensifies the hue provided by the specific alloy.
For standard yellow gold, 18K (75% gold) exhibits a deeper, richer yellow than 14K (58.3% gold). This is because the 18K piece contains a smaller percentage of diluting white and copper metals. Conversely, the 14K piece appears paler due to the greater presence of non-gold metals.
In colored alloys, the karat value affects the intensity of the alloy-driven color. For example, 14K rose gold has a higher percentage of copper than 18K rose gold, resulting in a more saturated reddish-pink color. Similarly, a lower-karat white gold alloy will have a more noticeable grayish or yellowish undertone, reflecting its lower pure gold content.
Superficial Color Changes: Plating and Treatments
The final color of gold jewelry is not always determined solely by the underlying alloy composition, as external surface treatments can temporarily alter its appearance. The most common treatment is rhodium plating, routinely used on white gold jewelry. White gold alloys naturally possess a faint yellow or grayish cast, which is not the brilliant white consumers expect.
Rhodium, a member of the platinum family, is a highly reflective metal electroplated onto the surface of white gold. This ultra-thin layer provides the bright white finish and adds a protective, scratch-resistant barrier. Because the plating is a superficial coating, it eventually wears away with time and friction, revealing the yellowish undertone of the white gold alloy beneath. Re-plating is then necessary to restore the bright color.
Standard gold plating is another common surface treatment, where a thin layer of gold is bonded to a base metal, which may or may not be a gold alloy. This process gives a less expensive metal the appearance of solid gold. Specialty colors, such as black or blue gold, are created not through the intrinsic color of the alloy, but through surface oxidation treatments or specialized patinas that chemically alter the metal’s outermost layer.