A hallmark is a small standardized statement, and the numbers in it have meanings that do not vary. The figure 925 means 92.5 percent silver. The figure 750 means 18 karat gold. The figure 585 means 14 karat gold. Alongside the fineness mark, a genuine piece carries a sponsor's mark naming the maker and, in many systems, an assay office mark and a date letter for the year. The whole stamp is a document, and like any document it can be checked against the public record.
Ask the seller for a sharp photo of the hallmark, clear enough to read, and compare the format against the assay office's published format: the shape of the punch, the letter for the year, the sponsor's mark. A stamp in the wrong font, crooked, or shallow where the genuine house stamps deep and straight, is a finding. A hallmark that decodes to a factory in one country for a piece the house only ever made in another is a finding. Counterfeiters copy the numbers. They copy the format badly.
Gold is dense, and that density is the hardest property to counterfeit. A gold chain of a given length and link size has a weight a buyer can predict, and a fake made of plated base metal weighs far less. Ask the seller for the weight, and compare it against a genuine piece of the same design. A chain that should weigh 40 grams and weighs 18 is not gold, whatever the stamp says.
The magnet test is the fastest physical check. Gold and silver are not magnetic, so a piece that sticks to a magnet is plated steel. It is one check in a stack, not a verdict alone, because some genuine pieces contain magnetic clasps or springs, but a chain that sticks along its length is not what it claims. The clasp deserves its own look: a genuine clasp closes securely, holds tension, and matches the metal of the piece. A clasp that feels flimsy or is a different color from the chain is a finding.
The price of gold and silver is public, and the weight of the piece is knowable, so a piece priced below the value of the metal it claims to contain cannot be what it claims. That arithmetic is the fastest check in the whole stack, and it is the one buyers skip most, because the bargain is the point. A gold chain at half the price of the gold in it is not a bargain. It is a piece with no gold in it.
The same logic applies to gemstones. A stone claimed as natural diamond or sapphire at a price that would not cover the cutting is glass or synthetic, and the setting is usually plated base metal. Ask for the stone's certificate where the listing claims one, and check the certificate number against the issuing lab. A certificate that does not verify is worse than no certificate, because it was printed to deceive.
Walk away from the listing, and report it with the specific discrepancy: the hallmark format does not match the assay record, the weight is inconsistent with the claimed metal, the price is below the metal value. A report that names the check is acted on faster than a report that says fake. Keep the evidence: the listing screenshots, the hallmark photo, the weight the seller gave, the price. That packet is what turns a takedown into a pattern, and patterns are what get a seller removed instead of relisted under a new name.
If this topic is useful, these earlier guides go deeper on connected questions: Counterfeit Perfume in Listings: Authentication Clues for Shoppers, Counterfeit Watches in Listings: Authentication Clues for Shoppers, and Serial Numbers Settle Arguments That Photos Cannot.
They are fineness marks: 925 means 92.5 percent silver, 750 means 18 karat gold, and 585 means 14 karat gold. A hallmark should also name the maker and, in many systems, the assay office and year.
Ask for a sharp photo of the hallmark and compare its format against the assay office's published format: the shape of the punch, the date letter, the sponsor's mark. A stamp in the wrong font, crooked, or shallow is a finding.
Gold and silver are not magnetic, so a chain that sticks to a magnet along its length is plated base metal. Some genuine pieces have magnetic clasps or springs, so test the chain itself, not only the clasp.
Price below the metal value proves the piece cannot be what it claims, because the metal price is public and the weight is knowable. A gold chain at half the price of the gold in it has no gold in it.