The honest answer to “is this real gold” is that you cannot be certain at home, and that this matters less than it sounds — because the checks you can do rule things out cheaply, and the one test that settles it takes a jeweller about thirty seconds and usually costs nothing.
What a stamp actually is
A purity mark is a claim. Who made the claim is the entire question.
In the UK, Ireland and much of Europe, an assay office physically tested the metal and struck its own marks alongside the maker’s. That is about as strong an assurance as exists anywhere in retail: an independent body with legal authority tested the actual object and put its name on the result.
In the US and most of the rest of the world, the maker struck the number themselves. It is illegal to lie, and the great majority do not. But nobody checked, and a stamp struck by an honest maker and a stamp struck by a dishonest one look identical to someone holding a ring.
There is a third case worth naming, because it accounts for a lot of confusion: marks that are entirely truthful and are read wrongly. 18KGP is a genuine, accurate mark meaning 18 carat gold plating. So is 1/20 12K GF, meaning a twentieth of the weight is 12 carat gold. Nobody has lied; the mark simply describes a layer rather than the item. Our hallmark reference sorts every common mark by what it actually claims.
Checks you can do without damaging anything
All of these share a property that is worth stating up front: they can prove a piece is not gold, and none of them can prove it is. That is not a weakness in the method — it is the shape of the problem. Passing every test below narrows things a great deal without closing them.
A magnet
Gold, silver, platinum and palladium are not magnetic. If a piece jumps to a magnet there is steel or another base metal in it — sometimes only in a clasp or a spring, so check where it sticks. If it ignores the magnet you have learned very little, because plenty of base metals are non-magnetic too.
Weight for its size
Density is the hardest thing on this list to fake. Gold is very heavy, and platinum is heavier still — around 40% more than 18K gold for the same volume. A ring that looks substantial and feels light is telling you something real. This is a feel test rather than a measurement, and it works better the more genuine pieces you have handled.
Wear at the high points
Plating wears through where the piece rubs: the underside of a ring shank, the edge of a clasp, the raised points of a chain. Look at those places under good light. A different colour showing through is plating gone, and it is conclusive.
Skin discolouration
Solid precious metal does not turn skin green. The reaction is between sweat and the copper in a base-metal core, which means the plating has failed somewhere. Note that solid low-karat gold contains copper too and can occasionally mark skin — so this is a strong hint rather than a verdict.
The stamp itself, magnified
Genuine marks are struck cleanly into the metal or laser-cut precisely. A mark that is blurred, uneven, oddly spaced, or sitting on the surface rather than in it is worth questioning. A phone camera at maximum zoom is enough to see this.
What this guide will not tell you to do
There is no acid test here. Acid testing genuinely works and jewellers use it daily — and it involves nitric acid, needs ventilation, and marks the piece. Handing those instructions to a stranger on the internet about their grandmother’s ring is not a good trade against the alternative, which is that almost any jeweller will test it in front of you, usually for free and usually with an XRF analyser that touches nothing.
The same goes for the “scratch it on a tile” and “bite it” tests that circulate. One damages the piece and the other tells you nothing that weight does not.
If it turns out to be plated
Then it has no recoverable metal value, and that is worth knowing before you drive anywhere. Refiners will not buy plated or gold-filled scrap — there is not enough gold present to be worth the chemistry — and a buyer who offers you something for it is buying it as jewellery, not as metal.
The one exception is vermeil, which is gold over sterling silver. The core is real silver, so a vermeil piece does have metal value, and it is silver value rather than gold value. You can put it through the silver calculator at 925.
And plated does not mean worthless. Plenty of costume jewellery is collected and traded well above any metal it contains. It simply means the question has moved from what it is made of to what it is.