K18 gold
K18 is the way Japanese jewellers mark 18 karat gold: gold that is 18 parts in 24, or 75 per cent, pure. It is the most common gold stamp on Japanese jewellery and watches, and with K24, K22, K14 and K10 it forms a family of marks that a collector learns to read at a glance.
Definition
Karats measure the gold in an alloy in parts per 24: 18 karat is 18 in 24, or 75 per cent gold, and 24 karat is considered pure gold. Millesimal fineness expresses the same thing in parts per thousand, so 75 per cent gold is written 750. Many European countries use the decimal stamps, such as 585 and 750, where the United States uses 14k and 18k. In Japan the karat is written with a K before the number: K18, K24. The remaining quarter of 18 karat gold is other metals; in our handling, yellow, pink and white K18 differ in that alloy, not in their gold content.
Origin
In the Edo period gold and silver were stamped with gokuin marks. In 1929, because dealers selling fakes were common, the Japan Mint, with its expertise in coinage and in refining and assaying gold and silver, began hallmarking under a Ministry of Finance ordinance. The Mint's hallmark certifies the purity of a piece; in our handling, many fine Japanese pieces carry only the maker's K mark.
Hallmarks beyond Japan
Hallmarking is one of the oldest forms of consumer protection. In France, the goldsmiths' rules issued in Paris in 1260 under Louis IX are an early example, and marks for silver followed by royal order in 1275. In England, a law of Edward I in 1313 required silver to meet the sterling standard, and the word hallmark comes from Goldsmiths' Hall in London, the home of the company chartered in 1327. Since 1973 a number of European countries have used a Common Control Mark under the Vienna Convention on hallmarking. In our reading, the Japan Mint's mark belongs to this family: an independent assay office vouching for the metal, separate from the maker's own stamp.
Key facts
| K18 | 18 karat, 75 per cent gold, 750 |
|---|---|
| K24 | Considered pure gold |
| Japan Mint hallmark | From 1929 |
| Colours | Yellow, pink and white K18 differ in their alloy, not their gold content |
How to identify it
- The stamp. K18 or 750, often with the maker's mark; on rings, inside the band.
- Combined marks. Pieces of two metals may carry both, such as K18 and Pt900.
- The Mint hallmark. A separate official mark beside the purity; our guide to Japanese gold and platinum hallmarks shows how to read it.
- Plating. In our handling, marks such as GP mean plated, not solid gold, and we state every mark exactly as struck.
Where it appears in the collection
Examples include a K18 yellow gold diamond half-eternity ring and a Bulgari Astrale ring in K18 yellow gold, on the jewellery shelf. Our essay on gold in Japan gives the longer history.
Related entries
- Pt900 platinum: the Japanese platinum mark
- Sterling silver 925: the silver standard
Sources
- Fineness, encyclopaedia entry, for karats, millesimal fineness and decimal stamps. wikipedia.org
- Hallmark, Japanese-language encyclopaedia entry (our translation), for French and English hallmarking history, the Common Control Mark, Edo-period marks and the Japan Mint's hallmarking from 1929. ja.wikipedia.org
Last reviewed 15 November 2026. The Japonista Curation and Concierge Team, Tokyo, Japan.