Gold Basics: Karat, Purity, Hallmarks and Weight Explained

Karat, Fineness, and Percentage Are Three Names for One Thing

Karat measures gold purity in twenty-fourths. 24K would be entirely gold; 18K is 18 parts gold and 6 parts other metals, so 75%. That is the whole system, and the rest is notation.

European and modern international marking uses fineness instead — parts per thousand. 18K becomes 750. 14K becomes 585. 22K becomes 916. Same alloys, different stamp, and if you have jewelry from more than one country you will meet both.

Two things worth internalising from that table. First, the drop is steeper than intuition suggests: 10K jewelry contains less than half the gold of 24K, gram for gram, so a heavy 10K chain can be worth less than a light 18K one. Second, the alloy metals are not filler in any pejorative sense — pure gold is soft enough to deform in a pocket, and the copper, silver and zinc in 14K are what let a ring survive being worn every day. Colour comes from the same place: rose gold is copper-heavy, white gold is alloyed with palladium or nickel and usually rhodium-plated on top.

The Unit Trap

Gold is priced per troy ounce, which weighs 31.1035 grams. A standard (avoirdupois) ounce — the one on your kitchen scale, the one in a pound of coffee — weighs 28.3495 grams. The troy ounce is about 9.7% heavier.

That difference is small enough to look like a rounding error and large enough to matter every time. If you weigh gold in ordinary ounces and multiply by the quoted per-troy-ounce price, you overstate your gold by roughly a tenth. People do this constantly, discover the shortfall at the counter, and conclude they were cheated when the arithmetic simply drifted.

The defence is to work in grams. Grams are unambiguous, every scale reads them, refiners quote in them, and the conversions out to other units are one multiplication when you actually need them:

• 1 troy ounce = 31.1035 g
• 1 standard ounce = 28.3495 g
• 1 pennyweight (dwt) = 1.55517 g — and 20 dwt = 1 troy ounce
• 1 kilogram = 32.1507 troy ounces

Pennyweight deserves a note because US jewelers still use it and almost nobody else does. If a counter quote is "per dwt," that is per 1.555 grams, so a dwt rate looks reassuringly larger than a gram rate for the same money. It is not a trick, but it is worth converting before you compare two quotes given in different units.

The troy and avoirdupois systems are both legally defined units maintained by the NIST Office of Weights and Measures, and UK purity marks are regulated by the British Hallmarking Council.

Reading the Stamps on Your Own Jewelry

Most jewelry carries its purity somewhere, usually somewhere awkward: inside a ring shank, on the flat tag beside a chain clasp, on the reverse of a pendant bail, on the underside of an earring post. A loupe or a phone camera at maximum zoom will usually find it.

What you will see falls into a few families. Karat marks — 10K, 14K, 18K, sometimes written 14KT or 585. Fineness numbers — three digits, occasionally four. Maker's marks, which are initials or a symbol and tell you who made it, not what it is. Assay office marks in the UK and parts of Europe, which are a genuine third-party guarantee. And the family that matters most for valuation: plating and bonding codes.

• GF or 1/20 12K GF — gold filled: a bonded layer, typically 1/20th of total weight
• RGP — rolled gold plate: thinner than gold filled
• HGE or HEG — heavy gold electroplate: microns
• GP — gold plated: thinner still
• Vermeil — gold over sterling silver; the silver has value, the gold layer barely does

If you see any of those, the item is not solid gold and a solid-gold calculator will overstate it by roughly twentyfold. That single distinction is the most valuable thing on this page.

What a Test Can and Cannot Prove

Home testing runs a spectrum from free to laboratory-grade, and each rung answers a narrower question than people assume.

The magnet test is free and only rules things out — gold is not magnetic, but neither is copper, brass, aluminium or lead, so passing proves very little. The hallmark is strong evidence in a hallmarking jurisdiction and weaker evidence elsewhere, since stamps can be forged. An acid test kit, about $15, is the first method that genuinely distinguishes karats, by seeing which acid strength dissolves a scratch of the metal — it is destructive in a tiny way and needs care. Specific gravity testing is non-destructive and surprisingly good, using nothing but a scale and a cup of water, but it is confused by hollow items and stones. XRF — the handheld gun a serious buyer will point at your item — is fast, accurate and non-destructive, and is the only one on this list that reads the actual alloy composition.

A practical rule: use the free tests to screen, buy the $15 kit if you have more than a couple of items, and let the buyer's XRF confirm before money changes hands. Ask to see the reading.

Then Do the Arithmetic

Once purity and weight are settled, the rest is one line:

grams × purity × price per gram = melt value

A worked example. A 14K chain weighing 25 g contains 25 × 0.5833 = 14.58 g of pure gold. At a spot price of $143 per gram, that is about $2,085 of gold.

That figure is the metal's intrinsic worth — objective, checkable, and independent of anybody's opinion. It is not what a buyer will hand you; every buyer takes a spread, and what each type pays is a separate question with its own honest answer. But melt value is the number that turns every offer into a percentage, and a percentage is something you can compare. Run yours on the gold calculator and the whole conversation changes shape.

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