Most of the tests circulating online either prove nothing or damage the piece. Three of them work, you can do all three in five minutes with things already in the house, and together they will tell you what you have with reasonable confidence.
The short version
Look for a 925 stamp, check it is not magnetic, and press it against ice — silver conducts heat so fast that ice melts visibly quicker than on any other metal. If all three agree, it is almost certainly sterling. The bleach test also works and permanently ruins the piece, so skip it.
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Test 1: the stamp
Sterling silver is stamped 925, sometimes S925, STER or STERLING. Look on the clasp, the inside of a band, or the back of a pendant.
These marks are often tiny — half a millimetre on a clasp — and worn stamps are the main reason people conclude a real piece is fake. A 30x jeweller's loupe with a light, around $23, makes the difference between reading a hallmark and guessing at it. If you buy secondhand jewellery at all, it pays for itself the first time.
What the numbers mean: 925 is 92.5% silver, the standard for jewellery. 999 is fine silver, too soft for most pieces. 800 and 835 are lower-grade European silver, still real. EPNS, silver plated or nickel silver mean no silver content worth counting — nickel silver contains none at all.
An absent stamp is not proof of anything. Older and handmade pieces often have none, and stamps wear off clasps over decades.
Test 2: a magnet
Silver is not magnetic. Hold a strong magnet — a fridge magnet is too weak, use one from a cupboard catch or a speaker — close to the piece. If it jumps or clings, it contains iron or steel and is not solid silver.
The caveat everyone misses: passing this test proves very little on its own, because plenty of non-magnetic metals are not silver either. Aluminium, brass and copper all pass. A magnet rules things out; it does not rule silver in.
Test 3: ice (the good one)
Silver has the highest thermal conductivity of any metal. Put an ice cube directly on the piece and another on a coin or a table for comparison. On silver the cube starts melting almost immediately and noticeably faster.
This is the most useful home test because it measures a property that is hard to fake, it costs nothing and it cannot damage anything.
Test 4: weight and sound, for the experienced
Silver is dense: a piece feels heavier than it looks. Tapping sterling produces a clear ringing note that lasts a second or so, while plated base metal gives a dull thud. Both tests are real but need a reference point — useful if you already own known sterling to compare against, and close to useless if you do not.
What not to do
The bleach test. A drop of bleach on silver turns it black quickly. It works, and it permanently tarnishes the spot in a way that is difficult to polish out. Do not do it to something you intend to wear.
Nitric acid tests. Accurate, and they leave a mark. Fine on scrap; not on jewellery.
Scratching it. Some guides suggest scraping a hidden area to see whether a different metal shows through. It does show through, and now you have a scratched ring.
The one that confuses everyone
A piece that turns your skin green is often assumed to be fake. It is not evidence either way. Sterling is 7.5% copper by definition, and copper is exactly what causes the green. The full explanation is here.
Likewise, tarnishing is a point in favour of silver rather than against it. Silver reacts with sulphur in the air and goes dark; stainless steel and most plated costume metals do not. A necklace that never tarnishes is more suspicious than one that does.
If you need to be certain
Any jeweller will test a piece with an acid kit or an XRF analyser, usually free or for a few pounds, and non-destructively in the case of XRF. Worth doing before buying anything expensive secondhand, and not worth doing for a twenty-pound pendant.
Related
How to clean tarnished silver — and why the method depends entirely on whether the piece is solid or plated.
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