Gold (Aurum, Au) is not uniform. While pure gold possesses a deep yellow color and exceptional resistance to corrosion, its inherent softness is a major limitation for practical applications. Pure gold must be modified to become durable enough for trade and manufacturing. The distinctions between different types of gold are determined by three factors: the amount of pure gold present, the specific metals it is mixed with, and the ultimate purpose for which it is intended.
The Standard of Purity: Karats and Fineness
The gold content in any object is precisely quantified using one of two widely recognized systems: karats or fineness. The karat system is the more traditional measure, dividing the total weight of the metal into 24 parts. Pure gold is designated as 24-karat (24K).
Lower karat values indicate a specific proportion of gold mixed with other metals, known as an alloy, to improve strength and reduce cost. For instance, 18-karat gold contains 18 parts gold and 6 parts alloy, resulting in a 75% gold concentration. Similarly, 14-karat gold is 58.3% gold (14/24), and 10-karat gold is 41.7% gold (10/24), which is often the legal minimum to be sold as gold in the United States.
The fineness system offers a more precise measurement, expressing purity in parts per thousand. This millesimal standard is favored in global trade and investment markets for its accuracy. A piece stamped with “750” indicates 750 parts per thousand of pure gold, which is the equivalent of 18K. The highest standard, often found in bullion, is “999.9,” representing 99.99% pure gold, though it is still technically referred to as 24-karat.
Differentiation by Alloying: The Spectrum of Gold Colors
Beyond purity, the color of gold is manipulated by the non-gold metals used in the alloying process. Alloying is necessary for durability and to introduce a spectrum of colors that pure gold cannot naturally achieve. The proportion of pure gold (the karat) can remain constant across different colors, while the type of additive metal dictates the final hue.
Traditional yellow gold is created by mixing pure gold with small, balanced amounts of silver and copper. This combination maintains the metal’s warm tone while increasing its hardness for jewelry. White gold is achieved by alloying gold with “white” metals such as nickel, palladium, or manganese, which bleach the natural yellow color. Because white gold alloys often retain a slight yellow cast, they are commonly plated with rhodium, a bright, silvery-white metal, to enhance their brilliance.
Rose gold, also called pink or red gold, owes its warm color to a higher concentration of copper in the alloy mixture. The increased copper content deepens the reddish hue, with a common 18K rose gold formula containing 75% gold and 25% copper. Less common varieties, such as green gold, are produced primarily by mixing gold with silver. The silver acts to lighten the yellow and introduce a subtle greenish tint.
Categorization by Purpose: Investment vs. Commercial Use
The intended application of the metal dictates the standards of purity and form it must meet. Investment-grade gold is primarily a store of value and is required to meet the highest fineness standards. Gold bullion, which includes standardized bars and specific coins, must have a minimum purity of 99.5% (995 fineness) to be accepted on major global commodity markets.
Most investment bars and coins are produced at 99.99% fineness, ensuring the buyer is acquiring the maximum amount of pure metal. This form is valued almost exclusively by its weight in troy ounces and its certified purity. The extreme softness of this high-purity metal is tolerated because it is not intended for heavy handling or decorative use.
Conversely, commercial and industrial gold is used in products where durability, strength, and conductivity are the main requirements. Jewelry relies on lower karats, such as 10K or 14K, because the higher percentage of alloy metals makes the final product resistant to scratching and bending during daily wear. Gold is also indispensable in electronics for its excellent conductivity and resistance to corrosion, where it is often applied in thin layers on contacts and connectors. Dentistry requires gold alloys with specific hardness and biocompatibility, often using 16K or 18K gold combined with platinum or palladium.