Diamond Tester and Gold Testing Kit Guide for Jewellers

An electronic diamond tester measures how quickly a stone conducts heat (and, on some models, electricity), which separates diamond from most imitations in seconds. A gold testing kit uses acid on a touchstone to show roughly what carat a piece is. Both are screening tools. They are fast and cheap, but neither one is final proof, and each has known blind spots.

This guide explains how both tools work, what they miss, and the habits that keep your results reliable. It is written for jewellers, pawn and gold buyers, and repairers who test stones and metal across the counter.

How does a diamond tester work?

Diamond is an unusually good conductor of heat. A thermal diamond tester has a fine metal probe tip that is kept slightly warm. When you press the tip onto a stone, a diamond pulls heat away from the tip quickly. The tester senses that drop and lights up or beeps. Glass, cubic zirconia and most other simulants pull heat away more slowly, so the tester reads "not diamond".

Many current testers also measure electrical conductivity, which is what lets them tell moissanite from diamond. Check which method a tester uses before you buy one, because that decides what it can tell you.

What a diamond tester can and cannot tell you

The point most buyers miss: a lab-grown diamond is real diamond, so a diamond tester cannot separate it from a natural stone. That takes laboratory screening equipment and, for important stones, a grading report. If a customer asks whether a stone is natural, a tester cannot answer that question.

How to test a diamond properly

1. Clean the stone. Oils, polish residue and dust insulate the surface and cause false negatives. Use a jewellery cleaner and dry it.
2. Let the tester warm up and check it. Follow the maker's instructions, and check the tester on a stone you know is diamond before you start.
3. Hold the probe square to the stone. Press the tip straight onto a flat facet. An angled probe touching an edge gives unreliable readings.
4. Keep the stone steady and the metal out of the way. A probe touching a prong or the setting can read the metal instead of the stone.
5. Test more than once. Take two or three readings on different facets. Let the tip recover between tests.
6. Treat odd results as a reason to look closer. Use a loupe to check for chips, coatings or glued-on pieces, and test the stone loose if you can.

Very small stones are the hardest to test, because the probe tip may be nearly as large as the facet. Pavé and melee work often need a tester designed for small stones, or a closer look with magnification instead.

How does a gold testing kit work?

A typical gold testing kit contains a touchstone (a dark, fine-grained stone) and a set of small bottles of test solution, each labelled with a carat value. You rub the piece on the touchstone to leave a streak, then put a drop of solution on it. If the streak stays, the metal is at or above that carat. If it fades or dissolves, the metal is lower. Comparing the streak against a known reference, such as a marked test needle, makes the result easier to read.

It works because lower-carat alloys contain more base metal, and acid attacks base metal faster than gold. It is quick and cheap, which is why it is the standard first check at the counter.

How to use a gold testing kit step by step

1. Look at the hallmark first. South African pieces are commonly marked in parts per thousand or carat, such as 375 (9ct), 585 (14ct), 750 (18ct) or 916 (22ct). A stamp is a claim to be tested, not proof.
2. Make a streak on the touchstone. Use a clean area and a firm, short stroke. Make a second streak with a reference piece of known carat if you have one.
3. Start with the lowest-carat solution. Apply a drop to the streak, then work upward. Watch the reaction for several seconds.
4. Compare the streaks. Where the unknown streak behaves the same as the reference, you have your carat.
5. Wipe the stone and wash your hands. Residue from one test will skew the next.

Gold testing kit safety

The solutions are corrosive. Wear gloves and eye protection, work in a ventilated space, keep bottles closed and upright, and keep them away from customers and children. Follow the supplier's storage and disposal instructions.

Where gold testing kits get it wrong

• Surface only. A scratch on the touchstone tests the outer layer. Gold plating or gold-filled material over base metal can pass. File or cut a small notch in an inconspicuous place (with the owner's consent) and test the exposed metal as well.
• Tungsten and other dense cores. Pure gold is about 19.3 g/cm³, and some fakes are made with dense cores to match its weight. They can feel right in the hand, which is why weight alone is not a test.
• Mixed pieces. Clasps, solder joints and repairs can be a different carat from the main body. Test more than one spot.
• Reading error. Colour changes are subtle between neighbouring carats, particularly with rose and white gold. Practise on pieces of known carat before you rely on your eye.

Electronic gold tester vs acid kit

Electronic gold testers measure electrical properties of the metal surface and display an approximate carat. They avoid handling acid and need no streak, so some buyers prefer them for high volumes or for finished jewellery they do not want to mark. They also read only the outer layer, and they need to be checked against samples of known carat. An acid kit costs less to start with but needs steadier technique. Many gold buyers keep both and use one to cross-check the other.

Add a weight check with a scale

A precise scale lets you work out density by weighing a piece in air and then in water. Compare the result with the expected density of the claimed alloy. It catches many dense-core fakes that pass a surface test, and it is a useful third check on bullion and larger pieces. For high-value purchases, send the item for assay or have it checked by XRF at a professional lab.

Buying a diamond tester or gold tester: what to look for

• Detection method. If moissanite is common in your market, choose a tester that checks electrical conductivity as well as heat.
• Stone size range. Check the minimum stone size the manufacturer states.
• Calibration and warm-up. Know how the tester is checked and how long it takes to be ready.
• Power. Decide whether batteries or mains power suit your counter, and keep spare batteries on hand.
• Kit contents. For a gold testing kit, check the carat range of the solutions, whether a touchstone and needles are included, and whether you can buy refills.

Where to get testers and scales in Pretoria

SM Watch Wholesale supplies the trade from No. 9 Bazaar Street, Marabastad, Pretoria, and stocks testers and scales next to jewellery cleaners, watch tools and parts. We sell to jewellers, repairers and retailers, not to the public, and trade customers sign in to see prices. You can read more about us or check delivery and collection options before you order.

Frequently asked questions

Can a diamond tester tell a lab-grown diamond from a natural one?

No. Lab-grown diamonds have the same thermal conductivity as natural diamonds, so a diamond tester shows both as diamond. Telling them apart takes specialised laboratory equipment.

Can a diamond tester tell moissanite from diamond?

Only if it also measures electrical conductivity. A thermal-only tester may read moissanite as diamond. Check the specification of the model before you rely on it.

Is a gold testing kit accurate?

It is accurate enough for a quick carat estimate on solid gold when you use it with care and compare against known references. It cannot see below the surface, so test a filed or cut spot on any piece you doubt, and use extra checks for high-value items.

Do I need a gold tester if the piece is hallmarked?

Yes. A hallmark tells you what the maker claims, and stamps can be faked or applied to plated items. Testing confirms the claim before you pay for it.

Where can I buy a diamond tester or gold tester for sale in South Africa?

Jewellery supply wholesalers and trade suppliers stock them. SM Watch Wholesale sells testers and scales to the trade. Sign in as a trade customer to see what is available and the prices.

Next step

If you repair watches as well as handle gold and stones, you may also need parts and small tools on the same order. Browse our watch spares and parts and sign in to see trade prices.