How to Test Gold

Quick Answer: How to Tell If Gold Is Real

The fastest reliable way to test gold at home is to work through four checks in order: read the hallmark stamp for a karat or fineness mark, run a magnet over the piece, weigh it and check its density against gold’s known 19.3 g/cm³, and — only if you still need an answer — use a karat acid testing kit on a small scratch mark. Each of these on its own can be fooled under the right circumstances, which is exactly why we run them as a stack rather than relying on any single result.

For anything beyond routine costume jewelry, the most reliable confirmation comes from equipment most people don’t have at home: an electronic conductivity tester, which reads a metal’s electrical properties through the surface without damaging it, and professional XRF analysis for surface composition. We use both at our counter every day, on top of the hallmark, magnet, and weight checks, because gold is valuable enough that sophisticated fakes exist and are worth ruling out properly.

What you should skip entirely are the folk tests that circulate online — toothpaste, vinegar, lemon juice, a lighter flame, or judging a scratch mark by eye alone. We cover exactly why each of those fails further down, but the short version is that none of them measures anything that actually distinguishes gold from a good imitation.

Start With the Hallmark: Reading Karat Stamps and Purity Marks

Before doing anything else, look for a stamp. Genuine gold jewelry is almost always marked somewhere discreet — inside a ring band, on a clasp, or on the underside of a pendant bail — with a karat number or a three-digit fineness figure. In the U.S., you’ll typically see 10K, 14K, 18K, 22K, or 24K. European and Asian pieces more often use the fineness number instead: 417 for 10K, 585 (sometimes stamped 583) for 14K, 750 for 18K, and 999 for 24K pure gold. These numbers describe the same purity scale two different ways — 14K and 585 both mean 58.3 percent gold by weight, with the remainder made up of alloying metals like copper, silver, or nickel that add strength and change color.

A stamp is a strong starting signal, but it is not proof by itself, because stamps can be forged, worn illegibly, or — more commonly — describe something other than solid gold. Watch for GP, GF, or HGE near a karat number: GP means gold plated, GF means gold filled (a thicker bonded layer of gold over base metal, more durable than plating but still not solid), and HGE means heavy gold electroplate. All three mean there’s a base metal underneath, and all three are legitimate, disclosed jewelry categories — the problem is only when that distinction gets lost or omitted by whoever is trying to sell you the piece.

We read the hallmark first on every item that comes across our counter, for the simple reason that it’s free, instant, and tells you what the manufacturer claims before any physical test even starts. It also flags plated and filled pieces immediately, saving a customer from wondering why a piece with a genuine-looking finish doesn’t behave like solid gold in the tests that follow.

If a piece has no stamp at all, that’s not automatically a red flag — older pieces, hand-made items, and gold from some countries are commonly unmarked — but it does mean you’ll be relying more heavily on the physical tests below rather than the manufacturer’s own claim.

The Magnet Test: What It Proves and What It Doesn’t

Gold is not magnetic — more precisely, it’s weakly diamagnetic, meaning it will not be attracted to a magnet at all under normal conditions. Run a reasonably strong magnet, like a small neodymium magnet, near the piece: if it snaps or pulls firmly toward the magnet, the piece contains a significant amount of a ferromagnetic metal such as iron, nickel, or steel, and it is not solid gold.

That makes the magnet test excellent for immediately ruling things out, but it can’t confirm gold on its own. A large number of non-magnetic metals and alloys — brass, aluminum, most stainless steel, and pure copper among them — will also show no response to a magnet, so passing this test only narrows the field. It’s one filter in the stack, not a verdict.

We treat a strong magnetic pull as a fast disqualifier and move on immediately when we see it, since there’s no need to run further tests on a piece that has just failed the simplest one. When a piece passes the magnet test, we treat that as permission to keep testing, not as confirmation the piece is gold.

Weight, Dimensions, and the Density Check

Gold is unusually dense — pure 24K gold measures 19.3 grams per cubic centimeter, which is nearly twice the density of common base metals used in cheap counterfeits, like brass or lead alloys. This is why a genuine gold coin or bar feels noticeably heavier in the hand than its size suggests, and it’s a check you can actually run at home with basic tools.

A simple version of this check uses water displacement. Weigh the item dry on a gram scale, then submerge it fully in a graduated container of water and record how much the water level rises in milliliters — that rise equals the item’s volume in cubic centimeters, since one milliliter of water equals one cubic centimeter of displaced volume. Divide the dry weight in grams by that volume, and you get the item’s density; a result close to 19.3 g/cm³ is consistent with high-karat gold, while a lower result points toward a lighter base metal or a hollow or plated piece.

For coins and bars, it’s often simpler to compare weight and diameter directly against the manufacturer’s published specifications rather than measuring density from scratch — an official mint sets exact weight and dimension tolerances for every coin and bar it strikes, and a piece that’s noticeably off on either measurement is worth a closer look regardless of how it looks or feels.

The one gap in this test, and it’s an important one for bullion buyers specifically, is tungsten. Tungsten’s density is about 19.25 g/cm³ — almost indistinguishable from gold’s 19.3 g/cm³ — which means a tungsten core with a thin gold shell around it can pass a casual weight-and-size check. We cover this in more detail in the counterfeit red flags section below, because it’s the single biggest reason we never rely on weight alone for bars and larger pieces.

Acid Test Kits: How Karat Acid Works — and How to Use It Safely

A gold acid test kit works by applying a small amount of nitric-acid-based solution, formulated at a specific strength for a specific karat, to a mark you’ve scratched onto an unglazed testing stone (sometimes called a touch stone). You scratch the item against the stone to leave a thin streak of metal on the surface, then apply a drop of the acid matched to the karat you’re checking — 10K acid, 14K acid, 18K acid, and so on — and watch what happens to the streak.

If the metal is at least the karat the acid is formulated for, the streak holds its color and doesn’t dissolve. If the true purity is lower than the acid’s strength, the streak will fade, bubble, or disappear entirely, which tells you the piece doesn’t meet that karat. Working through acids from lowest to highest karat lets you narrow in on the actual purity fairly precisely, which is why testing kits are sold with a full set of graduated strengths rather than a single bottle.

This is a genuinely destructive test in miniature — it leaves a small scratch and a chemical reaction on the item — so it should be reserved for cases where the hallmark, magnet, and weight checks haven’t given a confident answer, not used as a first step on a nice piece you plan to keep wearing. Always scratch and test in the least visible spot possible, such as the inside of a ring band.

Handle these acids the way the kit instructions require: wear gloves, work in a well-ventilated area or outdoors, keep the bottles capped when not in use, and neutralize or dispose of used acid according to the kit’s instructions rather than pouring it down a drain. Nitric acid solutions are corrosive and the fumes are unpleasant to breathe in an enclosed space, so basic precautions matter here even though the quantities involved are small.

Electronic Testers and XRF: How Professionals Verify Gold

Beyond what most people have at home, the tools we rely on daily are an electronic conductivity tester and, for higher-value pieces, X-ray fluorescence (XRF) analysis. An electronic tester like the Sigma Metalytics Precious Metal Verifier measures how a metal conducts electricity through its surface and compares that reading against known signatures for different metals and alloys. It’s non-destructive — no acid, no scratching — and it’s specifically good at catching one of the more sophisticated fakes in circulation: a tungsten-filled bar or coin with a genuine gold shell, which can pass a hallmark check, a magnet test, and even a rough weight check, but shows a conductivity signature that doesn’t match solid gold all the way through.

XRF works differently — it bombards the surface with X-rays and reads the characteristic fluorescence that different elements emit back, giving a precise readout of elemental composition. It’s the standard tool refiners and mints use for verification, and it’s excellent at confirming exact alloy composition non-destructively. Its limitation is one worth understanding honestly: XRF reads only the outer few microns of a surface, so a well-made plated or clad fake can, in principle, still return a misleading surface reading on XRF alone.

That’s exactly why professional verification doesn’t lean on any one method. We combine hallmark inspection, weight and dimension checks, a magnet pass, electronic conductivity testing, and visual inspection under magnification — because each method has a different blind spot, and a fake sophisticated enough to fool one of them rarely fools all of them at once.

We run this full process at our counter in San Antonio for every piece of gold that comes in, coins and bars included, and customers watch the whole thing happen in front of them rather than handing something over and waiting for a verdict.

Tests That Don’t Work: Toothpaste, Vinegar, Lemon, Lighter, and Ceramic Scratch

The toothpaste test — rubbing gold with toothpaste and judging it by how it looks afterward — doesn’t test purity at all. Toothpaste is a mild abrasive and cleaner, and rubbing almost anything with it will make a dull surface look shinier, whether that surface is 24K gold, gold-plated brass, or a stainless steel ring. A cleaner appearance afterward tells you the piece got cleaned, not what it’s made of.

Vinegar and lemon juice tests rely on the idea that acid discolors fake metal but leaves real gold unaffected. Both are weak, dilute acids, and in the short exposure times people actually use — a few drops for a few seconds — many base metals and even decent electroplating can shrug off the reaction with no visible change. That means a lack of reaction proves very little, and worse, a genuine lower-karat piece with a thin surface layer can occasionally show discoloration from the alloy underneath, creating a false negative on a real piece.

The lighter or torch test claims that real gold won’t blacken or melt under flame while fakes will. Gold’s melting point is 1,064°C (1,947°F) — nowhere close to what a household lighter or torch produces — so nothing definitive happens to genuine gold under a lighter regardless of purity. Any blackening people attribute to “fake” metal is usually the base alloy or plating oxidizing, but that reaction depends on the specific alloy, not on a clean gold/not-gold divide, and heat risks damaging solder joints, gemstones, and plated finishes in the process.

A scratch on an unglazed ceramic or touch stone, judged by eye color alone without following up with actual karat acid, is an incomplete version of the acid test rather than a real test on its own. Gold leaves a gold-colored streak, but so do several look-alike alloys and heavily gold-plated pieces, so the streak’s color tells you almost nothing until you add the acid step that the real test depends on.

Counterfeit Red Flags for Bullion Buyers — and Free Testing in San Antonio

If you buy gold bars or coins, the single most important fact to know is the tungsten coincidence: tungsten’s density of roughly 19.25 g/cm³ sits almost exactly on top of gold’s 19.3 g/cm³. That near-identical density is not an accident from a counterfeiter’s perspective — it means a tungsten core wrapped in a thin, genuine gold shell can pass a weight-and-dimension check that would catch almost any other fake metal, which is exactly why density and weight alone can never be the final word on a gold bar.

Sealed assay cards, the tamper-evident plastic packaging some mints and refiners use for smaller bars, add a layer of buyer confidence but aren’t foolproof — packaging can be replicated or reused around a substituted bar, so a sealed card is a good sign, not a guarantee. The strongest protection is simply buying from an established, verifiable dealer with a track record, rather than an anonymous online listing with a price that looks too good relative to the live spot market.

For live pricing context as you evaluate any purchase, our gold price page tracks the spot market throughout the trading day so you can see what gold is actually worth per troy ounce before you buy or sell.

Lone Star Coins tests gold for free, in front of you, every single day — hallmark inspection, the magnet test, calibrated scale weighing, acid testing when it’s the right tool, and Sigma Metalytics electronic verification for anything that needs the deeper check. There’s no appointment necessary and no obligation to sell; plenty of customers come in just to find out what they have. If you’d rather sell once you know, we buy gold jewelry, coins, and bars outright with same-day payment, and we ship nationwide for customers who can’t make it to our San Antonio showroom in person.