The karat is the unit by which the proportion of pure gold in a gold alloy is expressed. One karat is one part in twenty-four by mass, so 24-karat is theoretically pure gold (in practice 99.9 percent or finer), 22-karat is 22/24 or 91.67 percent gold, 18-karat is 75.0 percent, 14-karat is 58.3 percent, and 10-karat is 41.67 percent. The unit and its abbreviation (kt or K, with kt the more precise convention to distinguish it from the carat used for gemstone weight) are standard across most jewellery markets in countries that have not formally adopted the alternative millesimal fineness system.

Origin of the unit

The word derives from the Greek keration (carob seed), via the Arabic qīrāṭ, the same root that gives the gemstone carat. The historical link is to the use of carob seeds as small uniform mass standards in Mediterranean trade, and to the medieval Byzantine solidus, a 24-keration gold coin that established the 24-part division of gold fineness as a Mediterranean trade convention. The system survived into the European goldsmithing standards of the medieval period and was carried into the Anglo-American jewellery trade.

Standard fineness equivalents

Modern jewellery practice maps karat to millesimal fineness as follows: 24K = 999, 22K = 916, 21K = 875, 20K = 833, 18K = 750, 14K = 583 (commonly stamped 585), 10K = 416 (commonly 417), 9K = 375, 8K = 333. The European trade more commonly uses the millesimal stamp directly (750, 585, 375), while the North American, Indian, Middle Eastern and East Asian trades retain the karat designation. National standards specify how the relationship between stamped fineness and actual fineness is enforced, including legal tolerances.

Karat conventions by region

Different markets are dominated by different karat conventions for cultural and economic reasons. India and the Gulf are 22-karat markets — the high gold content reflects the bullion-as-savings function of jewellery in those economies. China, traditionally a 24-karat market for chuk kam pieces, has expanded into 18-karat for studded jewellery as the studded category has grown. The European fine jewellery market is principally 18-karat, with 9-karat used for lower-priced lines particularly in the United Kingdom and Ireland. North America runs 14-karat as the dominant fineness for wedding and bridal jewellery, with 10-karat at the lower end and 18-karat at the upper end. The plumb-karat regulations in the United States (Federal Trade Commission) require that the actual fineness equal or exceed the stamped value (with a 3-parts-per-1000 tolerance for unsoldered pieces and 7 parts for soldered).

Trade significance

The karat is foundational to gold jewellery trade documentation, hallmarking and international shipping. National hallmarking authorities — the BIS in India, the Assay Offices in the United Kingdom, the Bureau of Standards in various jurisdictions — administer testing and stamping regimes that translate the karat designation into a guaranteed minimum fineness. The disclosure standard for the contemporary trade is to mark by both karat and millesimal fineness on all but the smallest pieces.