Most buyers use an acid test, an electronic tester or an XRF machine. Acid and electronic tests estimate the karat; XRF reads the exact mix of metals at the surface without damaging the piece. Ask to see the result.
Why buyers test at all
A stamp tells you what a piece is supposed to be. It isn’t proof. Stamps wear off, get misread, and are sometimes faked, so a careful buyer tests before paying. The test result, together with the weight, decides the offer.
Knowing how the tests work helps you understand the number you’re given and spot a result that doesn’t add up.
The acid test
This is the oldest and most common test.
- The buyer rubs the piece on a dark test stone, leaving a thin streak of metal.
- They put a drop of acid made for a certain karat (for example, 14K) on the streak.
- If the streak stays, the gold is at least that karat. If it dissolves, it’s lower. They repeat with stronger acids to narrow it down.
What it’s good at: It’s quick, cheap and gives a karat range.
Where it can go wrong: It reads only the metal on the surface. A thick plating can pass as solid gold, so some buyers file a small notch first to reach the metal underneath. Ask before anyone files your piece. Reading the streak also takes some judgment, so two people can get slightly different answers.
Electronic testers
These handheld devices send a small electric current through a probe touching the piece, often with a drop of gel, and read how the metal responds. Different karats respond differently, and the screen shows an estimate.
What it’s good at: It’s fast, simple and leaves no mark.
Where it can go wrong: Like the acid test, it reads the surface, so it can be fooled by plating. Results can also be affected by how clean the piece is, its size and shape, and how the device is set up. A good buyer uses it as a quick check, not the final word.
XRF machines
XRF stands for X-ray fluorescence. The machine aims a small beam of X-rays at the piece, and the metals in it give off energy that the machine reads. It then shows the percentage of gold, silver, copper, zinc, nickel and other metals.
What it’s good at: It doesn’t damage the piece, gives an exact percentage rather than a range, and can show which other metals are present. That makes it useful for unmarked pieces and for telling white gold from platinum or silver.
Where it can go wrong: XRF only reads a very thin outer layer, a small fraction of a millimeter deep. A gold layer thicker than that over a different core can read as solid gold. Dirt, coatings and a curved surface can also affect the reading. XRF machines are expensive, so not every buyer has one.
Fire assay
Refiners use fire assay as the most accurate test. A small sample is melted and chemically separated to find the exact share of gold. It’s very precise but destroys the sample and takes hours or days, so it isn’t done at a shop counter. If you sell a large lot to a refiner, ask whether the final payment is based on an assay after melting.
Other checks you might see
- Magnet: Gold isn’t magnetic, so a strong pull points to steel or another base metal.
- Loupe: A magnifier to read the stamp and look for wear showing a different color underneath.
- Density: Weighing the piece in air and then in water shows how dense it is. Gold is much denser than most metals that imitate it.
Comparing the methods
| Test | Damages the piece? | What you get | Main weakness |
|---|---|---|---|
| Acid | Leaves a small streak, sometimes a notch | Karat range | Surface only; judgment needed |
| Electronic | No | Karat estimate | Surface only; setup and cleanliness matter |
| XRF | No | Exact metal percentages | Reads only a thin outer layer |
| Fire assay | Yes, the sample is destroyed | Exact gold content | Slow; done by refiners |
What to ask the buyer
- Can I watch the test? A reputable buyer will test in front of you.
- Can I see the result? Look at the acid streak, the tester screen or the XRF printout.
- What weight did you use? Ask for the weight in grams or pennyweight, and whether stones or steel parts were subtracted.
- Why is the result different from the stamp? A lower result can be honest, but ask them to explain it.
- Will you file or cut it? Say no if you plan to keep the piece or sell it as jewelry elsewhere.
Once you know the karat and weight, check the offer against the melt value in the gold calculator. See how to tell if a gold offer is fair and spot vs scrap price. To check a piece yourself first, see how to test gold at home.
If a piece turns out to be plated or gold-filled, see gold-filled vs gold-plated vs vermeil.