Gold and jewelry Practical guide
Gold karats, hallmarks, and how to calculate fine-gold weight
Read fineness marks such as 750, 875, and 999, and calculate how much fine gold a piece contains, including when it has stones.
Two pieces can weigh the same and contain different amounts of gold. An 18K item and a 21K item each contain a different proportion of gold in their metal. Reading the purity and the invoice gives you a way to compare gold content before comparing the price of the pieces.
Read karat as a fraction of 24
In 18K gold, 18 of 24 parts of the alloy are gold, which gives 75% by weight. The same calculation gives 87.5% for 21K. The US Federal Trade Commission explains that mixing gold with other metals affects hardness and its use in jewelry. Pieces with the same stated karat can have different alloy compositions while sharing the stated proportion of gold.
Some marks show fineness as parts per thousand. A 750 mark indicates 750 parts of gold in 1,000 parts of metal. A mark of 875 represents 87.5%. Use the documented fineness when precision matters. The fraction 14 ÷ 24 gives about 583.33 parts per thousand, while 585 is a fineness mark used under some standards. A high-purity product may state 999 or 999.9. Use the figure documented for the individual product.
| Fineness mark | Gold content by weight | Example description |
|---|---|---|
| 750 | 75% | 18K gold |
| 875 | 87.5% | 21K gold |
| 999 | 99.9% | High-purity gold |
| 999.9 | 99.99% | High-purity gold |
Distinguish the purity number from the recognized hallmark
A fineness number gives a description of the metal. A recognized hallmark may include marks for the maker, submitter, or testing body. UK guidance, for instance, describes a sponsor’s mark, a fineness mark, and an assay office mark as parts of its hallmark. Check local guidance when assessing a specific mark. Ask for the karat and type of metal on the invoice as well.
Read the full description when you see terms such as “gold plated.” A phrase such as “18K gold plated” can describe the purity of the surface layer alone. Estimating the gold content of a plated, composite, or older item can require more detailed documentation or examination by a qualified tester.
Calculate the fine-gold weight
Estimated fine-gold weight metal weight in grams × purity fraction.
Suppose a ring with a stone weighs 10 grams on the scale. The invoice identifies 8.4 grams as metal with 750 fineness. Multiply 8.4 by 0.75 to get an estimated 6.3 grams of fine gold. If you entered the entire 10 grams, the result would be 7.5 grams, an overestimate of 1.2 grams in this example. Ask for the metal weight separately when evaluating a piece with stones.
Compare pieces on the same basis
Consider a chain containing 7 grams of 21K metal and one containing 8 grams of 18K metal. The first holds about 6.125 grams of fine gold and the second holds 6 grams. The fine-gold content is close even though the weights and karats differ. Put the fine-gold weight and final price side by side, then consider the value of the design and making work. If the identity or purity of a piece is uncertain, have it assessed by a qualified tester before relying on the calculation.