Gold Testing Methods: How to Check Gold Purity at Home & Lab?

Gold purity can be checked using methods ranging from simple at-home tests (magnet, float, vinegar) to professional techniques (acid test, XRF, fire assay). For jewellery purchases or gold loans in India, verifying the BIS hallmark and HUID number is the fastest and most reliable check. Professional XRF testing offers the highest non-destructive accuracy.

Gold has always symbolised wealth, purity, and tradition in India. Whether you're buying jewellery, investing in gold, or verifying an inheritance, knowing how to check gold purity protects you from scams and helps you make informed decisions.

As fake and cheap gold items become more common, knowing all the different ways to test gold, from quick checks you can do at home to lab-grade methods, puts you in charge of your money.

This guide explains what gold purity actually is, the best ways to test gold at home and with experts, how each method compares in terms of accuracy and cost, and why purity matters whether you’re applying for a gold loan or planning to make a long-term investment.

Gold purity is expressed in two related systems: karats (K) and fineness. Pure gold is 24K, meaning it contains no other metals. Fineness expresses the same idea as a number out of 1000 — so 24K gold is also described as "999" fineness.

Gold is rarely sold in its purest form for jewellery because it's too soft to hold its shape under daily wear. Jewellers alloy it with metals like silver, copper, nickel, or zinc to improve strength and durability. This is why a 22K necklace and a 24K coin can look similar but behave very differently when bent or scratched.

In India, the BIS (Bureau of Indian Standards) hallmark is the official certification for jewellery. It indicates that the piece has been tested at a recognised assaying and hallmarking centre and meets its declared purity. Since 2021, hallmarked jewellery carries a HUID (Hallmark Unique Identification) number. This six-character alphanumeric code is unique to that specific piece and allows buyers to verify authenticity directly against BIS records rather than relying on the stamp alone.

A genuine BIS hallmark carries five components, each telling you something different about the piece:

1. The triangle-shaped BIS logo proves that the item was tested at a licenced facility.
2. The purity or fineness grade is shown by a number stamped on the metal, such as 916 for 22K or 750 for 18K.
3. The six-character HUID number is unique to that item.
4. The Assaying and Hallmarking Centre (AHC) mark lets you know which centre checked the item.
5. Jeweller's identification mark links the article back to the registered seller

To verify a hallmark, download the BIS Care app (available on Android and iOS), select "Verify HUID," and enter the six-character code engraved on your jewellery. The app pulls up the registered purity, the testing centre, and the hallmarking date directly from the BIS database. If the HUID doesn't return a match, treat that as a warning sign and get the piece independently tested before buying or pledging it.

Also Read: What Is the Major Difference Between 22K and 24K Gold?

Before reaching for a jeweller or a lab, a few simple checks can tell you whether a piece is worth a closer look. None of these at-home tests are conclusive on their own — they work best as a first screening step, not as proof of purity, especially before a large purchase, sale, or loan.

Pure gold is non-magnetic, so this is the simplest check available. Hold a strong neodymium magnet close to the piece; if it's drawn toward the magnet, the item likely contains iron, nickel, or another magnetic base metal rather than genuine gold.

Limitation: this test only rules out the most obvious fakes. Many counterfeit pieces use non-magnetic fillers like brass or lead, which will pass the magnet test despite containing no gold at all. A piece that doesn't react to a magnet is not automatically genuine — it has simply cleared the first, easiest hurdle.

Gold is one of the densest common metals, at roughly 19.3 g/cm³ — far heavier than most metals used to imitate it. Drop the piece into a glass of water: genuine solid gold sinks quickly and settles at the bottom, while gold-plated or low-purity fakes often float or sink more slowly.

This test works best on coins, bars, or solid pieces with no internal cavities. Hollow jewellery — common in lightweight chains and bangles — can trap air and give a misleading result. For a more precise reading, you can calculate the exact density by comparing the item's weight in air against its weight loss when submerged, which leads into the specific gravity calculation covered later in this guide.

Rub the piece firmly against your skin for a few seconds. Base metals and lower-purity gold alloys often react with skin oils and sweat, leaving behind a faint green or black mark caused by oxidation. Genuine high-karat gold is largely inert and typically leaves no mark at all.

Limitation: this test is unreliable for gold-plated items with a thick enough coating, since the base metal underneath never makes contact with your skin. It's a useful supporting check, not a standalone verdict.

Place a few drops of plain white vinegar directly on the gold surface and watch for a reaction. Genuine gold is chemically resistant and won't change colour or fizz. Lower-purity or fake gold often darkens, tarnishes, or shows visible discolouration within a minute or two.

This is a basic chemical indicator, not a purity measurement — it tells you whether something is obviously wrong, not what karat the piece actually is. Treat a clean result as a green light to proceed, not as confirmation of purity, and follow up with a professional test before any high-value transaction.

A note on at-home limitations: magnet, float, and vinegar tests are screening tools, not financial-grade verification. Each one can be fooled under specific conditions — a non-magnetic filler, a hollow setting, or a thick plating layer can all produce a false "pass". For anything beyond casual curiosity — buying jewellery, settling an inheritance dispute, or pledging gold for a loan — pair an at-home check with a certified professional test or a BIS hallmark verification.

When the stakes are higher — a large purchase, a loan appraisal, or a dispute over authenticity — professional testing methods offer a level of accuracy that at-home checks simply can't match. Jewellers, assay offices, and refineries rely on five main techniques.

The gold item is rubbed on a black touchstone to leave a visible streak, then a nitric acid solution is applied to that streak. The way the mark reacts — disappearing, discolouring, or holding firm — indicates the approximate karat. Jewellers typically keep separate acid strengths for 18K, 22K, and aqua regia (a combination of nitric and hydrochloric acid) for testing higher purities.

Best for: jewellers and small-scale gold buyers who need a fast, low-cost check.

This method is mildly destructive, since it leaves a small scratch and residue on the piece, and requires careful handling of corrosive acids. Accuracy also depends heavily on the operator's experience, and it only assesses the surface layer — a thinly gold-plated item can pass an inexperienced tester's acid test.

An electronic gold tester measures the item's electrical conductivity and displays an estimated karat value almost instantly. You simply place the probe against the gold surface and read the result on the device's screen.

Best for: retailers, collectors, and home use, where speed and a non-invasive process matter.

These devices are easy to use and require no chemicals, but they're not foolproof. Gold-plated items with thick coatings can return a falsely high reading, and the device needs periodic calibration to stay accurate.

XRF spectrometry fires X-rays at the gold's surface and measures the fluorescent energy each element emits in response, building a precise picture of the item's full elemental composition without damaging it. Accuracy typically runs to within ±0.1%, making it the benchmark for non-destructive testing.

Best for: certified assay offices, refineries, and large jewellers handling high-value transactions, where both precision and a non-destructive process matter.

The trade-off is cost: XRF equipment is expensive, which makes this method impractical for everyday personal use, even though it's widely accessible through certified testing centres for a per-item fee.

Fire assay is the oldest and most trusted purity test in the industry. A small sample is melted at high temperature along with lead and other fluxes; the mixture is then cupelled in a furnace, which oxidises and absorbs the base metals while leaving pure gold behind. The remaining gold is weighed precisely to calculate purity, typically to within ±0.01%.

Best for: gold refiners, official hallmarking centres, and dispute resolution, where absolute accuracy outweighs convenience.

It's the most accurate method available and is trusted by BIS and major assayers, but it destroys the sample in the process and takes considerably more time and money than any other test on this list.

This method uses the Archimedes principle to obtain a precise density reading without the need for specialised lab equipment. Weigh the piece in air using a digital scale, then weigh it again while fully suspended in a glass of water.

The formula is:

Specific Gravity = Weight in air ÷ (Weight in air − Weight in water)

Compare your result against known density values for common gold alloys (pure gold sits around 19.3 g/cm³; 22K and 18K alloys are slightly lower depending on the metals mixed in). This calculation is accurate for solid coins, bars, and dense jewellery, but it becomes unreliable for hollow or gem-set pieces, where trapped air or embedded stones can throw off the water-displacement reading.

If you're buying everyday jewellery, a BIS hallmark check plus an electronic or acid test at the jeweller's counter is usually sufficient. If you're pledging gold for a loan or settling a high-value transaction, ask for XRF testing — it gives near-laboratory accuracy without damaging the piece. Fire assay is generally reserved for refineries and formal disputes, where destroying a small sample is an acceptable cost for absolute certainty.

Gold purity is not merely about the quality of the metal. It has a direct impact on the worth of your gold, whether you are at the counter of a jeweller or at the valuation desk of a lender. When you apply for a gold loan, the lender will look at the purity and weight of your gold to determine the loan-to-value (LTV) ratio. In other words, it is the percentage of your gold's market value that lenders will give you as a loan.

Higher-purity gold generally qualifies for a better LTV ratio. 22K jewellery — the most common purity for Indian gold jewellery — is widely accepted by lenders and tends to fetch a higher eligible loan amount. 18K gold, which contains a larger share of alloyed metals, typically attracts a lower valuation since less of the pledged weight is actual gold.

The same principle applies to long-term investment. Purity directly affects resale value, since buyers and dealers price gold per gram based on its actual gold content, not its total weight. BIS-hallmarked gold tends to command better resale prices because the certified purity removes any guesswork for the next buyer.

If you're preparing gold for a loan appraisal, a few simple steps help the process move faster: keep the original purchase invoice and hallmark certificate handy; avoid mixing gold and non-gold components (like gemstones or enamel work) where possible, and get an independent purity check beforehand if your jewellery predates the HUID system, so you have a realistic expectation of its appraised value.

A few warning signs are worth checking before you buy, sell, or pledge any gold item:

• Discolouration at edges or clasps: These high-friction points wear down fastest, exposing the base metal underneath a thin gold plating.
• Unusual lightness for its size: Gold is dense; a piece that feels surprisingly light for its volume may be hollow or made from a lighter base metal entirely.
• Green or black marks on skin: A reaction with skin oils points to base metals reacting with sweat, which pure or high-karat gold typically doesn't do.
• No BIS mark or HUID: In case jewellery is being sold without any BIS mark or HUID, there is no official assurance of purity, regardless of what the seller claims.
• Price significantly below market rate: Gold trades close to its live market price; a deal that looks too good is usually a sign of inflated karat claims or undisclosed alloying.
• Strong reaction to a magnet: While not conclusive on its own, visible magnetic attraction is one of the clearest signs that a piece contains iron or another magnetic base metal.

The most reliable safeguard against all of these is buying only from certified jewellers and verifying the BIS hallmark and HUID before completing any purchase.

To make smart financial decisions — whether you're investing in gold or planning ahead — it helps to partner with a reliable financial provider. Explore safe and flexible gold investment options with Aditya Birla Capital. From gold loans to broader money management, Aditya Birla Capital offers solutions built around your financial goals.

Also Read: Different Forms of Gold: Types of Gold, Colours, and Carats