Gold, designated by the chemical symbol Au, is one of the few metals that displays color in its pure form, appearing as a slightly reddish-yellow metal. This unique hue results from relativistic effects on its electrons, causing the absorption of light in the blue spectrum. However, 24-karat gold is naturally soft, making it impractical for everyday use. To increase its hardness and durability, gold must be mixed with other metals, a process known as alloying, which dramatically changes the gold’s color.
The Foundation: Pure Gold and Alloying
The characteristic yellow color of pure gold stems from the movement of its electrons between energy bands when light strikes its surface. Unlike most other metals, which reflect all visible light wavelengths equally and therefore appear silver or gray, gold absorbs a small portion of blue light. The remaining reflected light mixture, which is deficient in blue, is interpreted by the human eye as yellow.
The process of alloying intentionally disrupts this natural electronic structure by introducing foreign atoms. When metals like copper, silver, or palladium are mixed with gold, their atoms integrate into the gold’s crystal lattice. This blending alters the electron energy levels, shifting the wavelengths of light the new alloy absorbs and reflects.
The resulting color of the alloy is primarily determined by the type and percentage of the secondary metal added. Alloying increases the gold’s mechanical strength and expands the visual spectrum it can display.
Common Jewelry Colors and Their Metal Recipes
The most common colors found in fine jewelry are achieved through precise ratios of gold, copper, and silver. These combinations result in alloys that are both aesthetically pleasing and sufficiently durable for daily wear.
White gold is created by combining pure gold with “whitening” metals such as palladium, nickel, silver, and zinc. The high percentage of these white metals counteracts the natural yellow, often resulting in a pale gray or faint yellow hue. Because the alloy is not a true bright white, most white gold jewelry is finished with a layer of rhodium plating. This plating provides the brilliant, silvery finish consumers expect but eventually wears away, requiring periodic re-plating to maintain luster.
Rose, pink, and red gold are all produced by alloying gold with copper, the only other pure metal besides gold to exhibit color. The copper imparts a warm, reddish tint, and the specific hue depends entirely on the copper concentration. For instance, a typical 18-karat rose gold contains 75% gold, with the remaining 25% composed mostly of copper and a small amount of silver to soften the tone. Red gold, by contrast, contains a higher proportion of copper and often eliminates silver entirely, resulting in a deeper, more saturated crimson shade.
Green gold, historically known as electrum, is a naturally occurring alloy of gold and silver. When silver is added to gold, it yields a subtle greenish-yellow color. While less common in modern jewelry, it can also be created by adding small amounts of zinc.
Exotic and Chemically Complex Hues
Beyond the common alloys, certain colors are achieved through more complex chemical processes, often yielding materials that are visually striking but less practical for everyday jewelry. These hues often rely on the formation of intermetallic compounds or specialized surface treatments.
Blue gold is produced by alloying gold with either indium, gallium, or iron. The gold atoms chemically bond with the alloy metal to form an intermetallic compound with a distinct crystal structure. The color results from how this new structure interacts with light, but the compounds formed are inherently fragile and brittle.
Similarly, purple gold, also called amethyst gold, is an intermetallic compound formed when gold is alloyed with aluminum, typically at a ratio of 75% gold to 25% aluminum. The resulting material, gold aluminide, is a beautiful violet color but is so brittle it can shatter if struck sharply. For this reason, blue and purple golds are rarely used for full jewelry pieces and are instead incorporated as delicate inlays or accents.
Black gold is not achieved through a bulk alloy but rather through surface treatments. One method involves using a black rhodium or ruthenium electroplating process, which deposits a thin, durable black layer onto the gold surface. Another technique utilizes controlled oxidation of gold alloys containing elements like cobalt or chromium, where heat treatment causes a black oxide layer to form on the metal’s exterior.
How Color Relates to Karat Purity
The Karat system provides a measurement of gold purity, which is directly linked to the intensity of the alloy’s color. The system is based on a fraction of 24 parts, meaning 24-karat gold is 24 parts gold, or 99.9% pure.
Any alloy below 24-karat indicates that a portion of the gold has been replaced by other metals. For example, 18-karat gold is 18 parts gold and 6 parts alloy (75% pure), and 14-karat gold is 14 parts gold and 10 parts alloy (58.3% pure). As the karat number decreases, the percentage of the coloring metal increases, leading to a more saturated color.
This principle means that 10-karat rose gold, which contains the lowest percentage of gold allowed (41.7%), will have a significantly higher copper content than 18-karat rose gold. Consequently, the 10-karat piece will display a much deeper, redder hue. Conversely, 18-karat white gold will appear more yellow than 14-karat white gold because its higher gold content is more difficult to counteract with the white alloying metals. The karat stamp serves as both a purity marker and an indicator of color saturation.