How to Check Gold Purity
The most reliable everyday check is the hallmark: in India a BIS logo, purity grade such as 22K916 and a six-character HUID code you can verify in the BIS CARE app. If a piece has no hallmark or you doubt it, pay for a non-destructive XRF test at a hallmarking or assaying centre. Home tests such as magnets and acid kits can expose obvious fakes but cannot confirm karat reliably.
Step 1: read the hallmark
A hallmark is an independent test result stamped on the metal. In India, jewellery sold by a registered jeweller must carry the BIS hallmark, which since 1 April 2023 has three parts: the BIS triangle logo, the purity in karat and fineness (for example 22K916, 18K750, 14K585) and a six-digit alphanumeric HUID. Enter the HUID in the BIS CARE app to see the jeweller, purity and item type recorded when it was tested. If the app result does not match the piece, do not buy.
Elsewhere, look for fineness numbers (999, 916, 750, 585, 417, 375) or karat marks (24K, 22K, 18K, 14K, 10K, 9K). In the UK, hallmarking by an assay office is legally required above set weight thresholds. In the US there is no mandatory third-party hallmark: a karat stamp must be accurate and is usually accompanied by the maker's trademark, but no independent office tests it. Use our karat guide and 916 explainer to decode the numbers.
Step 2: professional testing
| Method | Where | Damages the piece? | What it tells you |
|---|---|---|---|
| XRF (X-ray fluorescence) | Hallmarking and assaying centres, many jewellers and buyers | No | Surface composition in a minute or two; very good for solid items |
| Fire assay | Assay offices, refiners | Yes, a small sample is melted | The reference method; exact fineness of the whole sample |
| Touchstone | Traditional jewellers, pawnbrokers | Minor surface rub | Approximate karat from comparing streaks with test needles and acids |
| Electronic testers | Dealers | No | Approximate karat; reads the surface only |
XRF reads only the outer few microns, so a thick plating over a base metal or a tungsten core can fool it. Reputable buyers combine XRF with weight, density and, where in doubt, a small cut or drill sample. In India, BIS-recognised assaying and hallmarking centres will test a customer's own jewellery for a small fee, which is useful before selling or when buying from an individual.
Home tests and their limits
- Magnet test. Gold is not magnetic. If a strong magnet pulls the piece, it contains iron, nickel or a steel core. But passing proves little: copper, brass, silver, aluminium and tungsten are not magnetic either, and clasps or springs may contain steel legitimately.
- Acid test kit. A streak on a black stone is treated with acids rated for each karat; the streak dissolves if the gold is below that grade. It gives a rough karat but only for the surface, and nitric acid needs careful handling.
- Density (water displacement). Pure gold has a density of about 19.3 g/cm³; 22K is lower because of the alloy. Weigh the piece, then measure the water it displaces. The test is useful for plain coins and bars, but tungsten (about 19.25 g/cm³) is almost identical to pure gold, and hollow or stone-set jewellery gives meaningless results.
- Skin or ceramic streak tests. Discoloured skin or a black streak on unglazed ceramic suggests base metal, but alloys, cosmetics and lighting make these unreliable.
None of the home tests can tell 22K from 20K, or a well-made gold-plated tungsten bar from a real one. Use them as a first filter, never as proof before a large purchase or sale.
Plated, filled and vermeil: what the terms mean
| Term or mark | Meaning | Melt value |
|---|---|---|
| Solid gold (e.g. 14K, 585) | The alloy is gold of that fineness throughout | Full value for the gold content |
| Gold-filled (GF, e.g. "1/20 14K GF") | US term: a layer of karat gold bonded to a base metal, at least 1/20 of the total weight | Low; a small fraction of a solid piece |
| Rolled gold plate (RGP) | Like gold-filled but with a thinner bonded layer | Very low |
| Gold-plated, GP, GEP, HGE | A thin electroplated layer over brass, copper or steel | Negligible |
| Vermeil | Sterling silver plated with gold; US guidelines require at least 10K gold and a meaningful thickness | Mostly the silver value |
A mark like "18K GP" or "14K HGE" can mislead at a glance: the karat refers only to the plating. Buyers price such items as base metal or silver.
What not to do
- Do not bite it. Pure gold is soft enough to dent, but so is lead, and 22K or 18K alloys will not mark. You only damage the piece and your teeth.
- Do not scratch or file deep marks to look for a different colour underneath. It ruins resale value, and buyers will deduct for damage.
- Do not use a lighter or flame. Heat can discolour alloys, damage stones and does not reveal plating reliably.
- Do not rely on colour. Alloy metals change the shade: rose gold has more copper, white gold is often rhodium-plated, and a deep yellow can be achieved by plating.
Purity and price
Purity sets the value. A 10 g chain stamped 22K holds 9.16 g of fine gold; the same weight in 18K holds 7.5 g. Once you know the true fineness, enter it in the karat purity calculator or the scrap gold calculator to see what the metal is worth at the live rate.