K18 is the Japanese marking convention for 18-carat gold, indicating an alloy of 750 parts gold per 1,000, equivalent to the European 750 fineness mark and to the British 18ct hallmark. It is the most common gold-fineness mark used in Japanese jewellery production and is also widely seen in jewellery exported from Japan to Hawaii, the United States West Coast and other markets with significant Japanese consumer presence.

Marking convention

Japanese gold marking uses the K prefix, where K is the Japanese rendering of carat. The number after the K is the carat value, so K24 is 24-carat or pure gold, K22 is 22-carat or 916 fine, K20 is 20-carat or 833 fine, K18 is 18-carat or 750 fine, K14 is 14-carat or 583 fine, and K10 is 10-carat or 417 fine. The marking is self-applied by the manufacturer rather than struck by a state assay office, in contrast to British, French and Italian hallmarking practice.

Production characteristics

K18 alloy in Japanese jewellery is produced in yellow, white, pink and rose colour variations, with the alloy chemistry chosen for colour, hardness and resistance to tarnish. Japanese white K18 is typically alloyed with palladium rather than nickel, in line with European practice and in part because Japanese consumers have historically been sensitive to nickel-related contact dermatitis. Japanese pink and rose K18 alloys typically include copper and small amounts of silver to produce a softer pink hue than the more saturated rose tones seen in some Italian production.

Trade significance

K18 is the standard Japanese fineness for fine jewellery and is the marking found on the great majority of Japanese-produced bridal sets, diamond jewellery, designer pieces and watch cases. For the international trade, K18 is interchangeable with European 750 markings under the FTC Jewelry Guides and the British, French, Italian, German and Swiss hallmarking conventions, although a separate import-marking step may be required in jurisdictions with state assay offices. K18 markings are commonly encountered in the second-hand and estate trade in Hawaii, California and other Japanese-tourist-influenced markets.