Gold purity, expressed as a karat or hallmark number, indicates the amount of pure gold contained in a jewelry piece and is the key factor determining its value. The most common hallmarks are 375, 585, 750, and 999 — the higher the number, the greater the proportion of pure gold in the item.
Several methods are used to determine purity:
- Hallmark inspection — every genuine gold item should carry an official stamped mark indicating its purity. This is the simplest first check.
- Acid testing — nitric acid of varying concentrations is applied to the item's surface, and the purity is estimated based on the reaction.
- Electronic gold testers — these devices measure electrical conductivity and provide a fast, non-destructive reading.
- X-ray fluorescence (XRF) analysis — the most accurate laboratory method, giving a precise composition percentage without damaging the item.
- Touchstone method — a traditional technique where the mark left by gold on a touchstone is compared by color to estimate purity; jewelers have used this for centuries.
When buying gold, it's always advisable to check the hallmark and, where possible, ask for a certified lab report.
