How to Test Gold Purity Without Destroying It

A karat stamp tells you what the manufacturer says a piece is. A purity test tells you what it actually is. If you're buying scrap gold — especially from the public, at pawn shops, or on Facebook Marketplace — the difference between those two things can cost you hundreds of dollars on a single deal.

Fortunately, you don't have to destroy a piece to find out what's inside it. Multiple non-destructive testing methods exist, ranging from free to professional-grade, and every serious buyer should have at least two of them in their toolkit.

Here's how each method works, what it catches, and where it falls short.

The Magnet Test

Cost: $5–10 for a neodymium magnet. Accuracy: Low — rules out obvious fakes.

Gold, silver, platinum, and copper are not magnetic. Hold a strong neodymium magnet (not a fridge magnet — you need a rare earth magnet) against the piece. If it sticks, the piece contains a significant amount of ferromagnetic metal and is not solid gold.

This test takes two seconds and catches the worst offenders — gold-plated steel, gold-plated iron, and some counterfeit pieces. It will not catch gold-plated copper or brass, because those base metals aren't magnetic either. It also won't detect the difference between 10K and 18K gold, since both are non-magnetic.

Use the magnet test as a first screen, not a final answer. If a piece sticks to the magnet, it's not solid gold — don't buy it. If it doesn't stick, you need further testing before you can be confident in the karat.

Note: some clasps and springs on genuine gold jewelry use base metal components. Test the main body of the piece, not just the clasp.

The Acid Test

Cost: $15–30 for a kit. Accuracy: Moderate to good — identifies karat ranges.

Acid testing is the traditional jeweler's method. A gold acid test kit contains several bottles of acid, each formulated for a specific karat. You scratch the gold piece on a testing stone (leaving a small streak of metal), then apply the corresponding acid. The reaction — whether the streak dissolves, changes color, or stays intact — tells you the approximate karat.

How it works in practice: scratch the piece on the stone, apply 14K acid. If the streak stays, it's at least 14K. If it dissolves, it's below 14K — try the 10K acid. If the streak stays with 10K acid, it's 10K to 14K. Then try 18K acid to narrow it further.

Pros: Cheap. Portable. Reliable for distinguishing solid gold from plated metal. Can differentiate karat ranges (10K vs 14K vs 18K vs 22K).

Cons: Technically not 100% non-destructive — the scratch on the testing stone removes a tiny amount of material, though it's negligible and usually invisible on the piece. The acids are corrosive and have a shelf life. Accuracy depends on technique — shallow scratches give unreliable results. Won't give you an exact purity percentage, only a range.

Acid testing is the best value method for most scrap gold buyers. Learn to do it properly and it'll catch most misrepresented pieces.

Electronic Testers

Cost: $150–500+ depending on quality. Accuracy: Good — gives karat reading in seconds.

Electronic gold testers measure electrical conductivity or resistance to determine gold content. You apply a gel or solution to the piece, touch the probe to the surface, and the device displays a karat reading. Some models test through multiple layers, which helps identify thick plating.

Pros: Fast — results in seconds. Non-destructive. Easy to use in the field. No chemicals to carry. Gives a specific karat reading rather than a range.

Cons: Quality varies dramatically by brand and price. Cheaper models can be fooled by certain alloys or thick plating. Surface treatments, heavy patina, or dirt can affect readings. Requires calibration and gel or solution refills. Not as reliable on very small or very thin pieces.

Electronic testers are a good middle-ground between acid testing and XRF. They're faster than acid in the field and accurate enough for most scrap buying. Just don't rely on a cheap one as your only test.

XRF Analysis

Cost: $5,000–30,000+ for a handheld unit, or $25–50 per test at a jeweler. Accuracy: Excellent — precise purity percentage.

X-ray fluorescence (XRF) analysis is the professional standard. A handheld XRF gun fires X-rays at the piece and analyzes the energy returned by each element. It gives you an exact breakdown of every metal in the piece — 57.82% gold, 24.15% copper, 12.03% silver, etc.

Pros: Most accurate non-destructive method available. Gives exact purity down to decimal points. Identifies every metal in the alloy. Can't be fooled by surface treatments. Works on any size or shape piece.

Cons: Extremely expensive. Handheld units cost thousands. Even used or refurbished guns are a significant investment. Requires safety training (X-ray device). Overkill for casual buyers.

If you're buying scrap gold at volume — dozens of deals per week — an XRF analyzer is a business investment that pays for itself by catching every karat discrepancy. Some scrap buyers recoup the cost within the first year by identifying pieces that test higher or lower than stamped. Nu Stack's Calculator supports XRF custom purity override, so you can enter the exact percentage from your XRF reading instead of selecting a standard karat — your melt value calculation is then based on exactly what the piece contains.

For lower-volume buyers, some jewelers and coin shops will XRF test pieces for a small fee. Worth doing on high-value purchases where the cost of being wrong is significant.

The Archimedes Density Test

Cost: $20–30 for supplies (scale, water, cup). Accuracy: Good for identifying gross misrepresentation.

The Archimedes method uses the principle that pure metals have a known density. Gold is extremely dense — 19.3 g/cm³. Most base metals are significantly less dense. By weighing a piece dry and then submerged in water, you can calculate its density and compare it to the expected density for its claimed karat.

The process: weigh the piece on a dry scale (record as dry weight). Suspend the piece in a cup of water sitting on the scale — the difference between the wet and dry readings, combined with the dry weight, gives you the density. Compare that density to known gold alloy densities.

Pros: Completely non-destructive. No chemicals. Equipment is cheap — any accurate scale and a cup of water. Based on physics, not chemical reactions.

Cons: Less precise than XRF or acid testing. Works best on solid pieces without hollow sections, stones, or mixed-metal components — hollow pieces will give artificially low density readings. Requires a scale accurate to 0.01 grams for best results.

Nu Stack's Weight Assistant (Pro tier) automates the Archimedes calculation — enter your dry and submerged weights and it estimates purity based on the density result. It's a useful supplementary test, especially for bars and solid pieces where the density measurement is most reliable.

Which Methods Should You Use?

For most scrap gold buyers, carry at least two methods. The recommended starter combination is a neodymium magnet plus an acid test kit. Total cost: under $40. This catches fake gold (magnet) and identifies karat ranges (acid).

As your volume increases, add an electronic tester for speed in the field and consider XRF access for high-value deals.

No single test is perfect. Layer your testing — use the quick methods first to screen, then use more precise methods to confirm on pieces worth significant money.