Karat measures a proportion
The karat system expresses gold purity in 24 parts. The theoretical pure-gold ratio is karat divided by 24. Fineness expresses a similar proportion in parts per thousand.
- 24K: 24/24, approximately 999 or higher fineness when sold as high-purity bullion
- 22K: 22/24, approximately 916.7 fineness
- 21K: 21/24, 875 fineness
- 20K: 20/24, approximately 833.3 fineness
- 19K: 19/24, approximately 791.7 fineness
- 18K: 18/24, 750 fineness
Purity is not the whole product
Alloy metals can improve hardness, colour or workability. A lower-karat item is not automatically poor quality; it may be designed for durability. The important point is that the invoice, stamp, test and price all describe the same purity.
Verify before paying
Ask for the stated karat, net gold weight and testing method in writing. A stamp is useful evidence but should not replace an invoice or an appropriate test when the value is material.