Gold is the most coveted metal in the history of mankind. Since ancient times people have been fascinated by the metal’s beauty and fitness for jewelry use—its malleability as well as its resistance to corrosion, oxidation, and scratching. Examples of elaborate gold workmanship have survived from early civilizations spanning the globe: Incan, Mayan, and Aztecan in South America and Central America; Egyptian, Minoan, and Assyrian civilizations of the Middle East; the Etruscan civilization covering the territory of modern Italy; Harappan in India; and Sarmatian in the southern Ural mountains.

Today gold in its pure form is rarely used to produce jewelry. Most 24-karat gold is sold as investments to world governments and central banks as well as private investors. Twenty-four-karat gold is too soft and not durable enough to use for jewelry. Jewelers use different alloys, combining gold with other metals to give it more hardness. For example, the most popular grade of gold purity in the United States is 18-karat, meaning three-quarters of the alloy is gold and one-quarter is a mixture of copper and silver.
Mixing in silver and copper is a bit of science—if too much silver is added, the gold will show a green tinge. On the other hand, mixing in too much copper will give gold a red tinge (so-called “rose gold”). Addition of nickel or palladium overpowers the distinctly bright yellow color of gold, producing what is known as “white gold.” Mixing other metals with gold produces a variety of colored golds, such as green gold , pale yellow gold , yellow gold, red gold, and white gold.

Gold purity is measured in karats (known as “carats” in countries of the British Commonwealth) and refers to the amount of pure gold present in the jewelry. Twenty-four-karat gold is considered to be pure gold, or 99.9% pure to be exact. Dividing the karat unit by 24 will provide the percent of pure gold in the jewelry. Thus, if you are wondering how much gold is in the 18-karat wedding band you are wearing, divide 18 by 24 to get 75%. Your wedding band gold is 75% pure.

In Europe gold purity is expressed using millesimal fineness, parts per thousand of pure metal by mass in the alloy, rounded to three digits. Gold rated 24-karat or 99.9% pure in the United States would be stamped “999” in Europe.

The table below provides translation of the U.S. gold purity system into the European one.

Gold Content and Notation

USA Karat Stamping	Parts Gold	Gold %	European Gold Purity Stamp
24K	24 / 24	99.9%	999
22K	22 / 24	91.7%	916 or 917
18K	18 / 24	75.0%	750
14K	14 / 24	58.3%	583 or 585
12K	12 / 24	50.0%	500
10K	10 / 24	41.7%	416 or 417
9K	9 / 24	37.5%	375

Stating the obvious, the purer the gold content in a piece of jewelry, the more valuable it is. Thus, with the current price of gold hovering around $900 per troy ounce (equivalent to 31.103 grams) the price difference between 24-karat gold and 18-karat gold would be $225 per troy ounce. One troy ounce of 18-karat gold would cost 75% of $900, or $675.

Jewelers use special X-ray fluorescence instrumentation to test the karat grade of precious metals and jewelry. These machines determine the gold karat measurement without damaging fine jewelry. More sophisticated equipment can also determine proportion of gold, platinum, silver, palladium, rhodium, and other precious and semi-precious metals in the alloy composition of gold jewelry.