Last Updated: April 2026
TL;DR:
- A diamond tester works by measuring how fast heat (and in newer models, electricity) travels through a stone. Diamond conducts heat far faster than glass, cubic zirconia, or most simulants, and the tester reads that difference.
- Heat-only testers cannot separate diamond from moissanite — both have very similar thermal conductivity. To tell those two apart you need a tester that also measures electrical conductivity.
- Diamond testers cannot distinguish lab-grown from mined diamonds, because the two are chemically and physically identical. The tester confirms a stone is diamond, not where it came from.
A diamond tester is the fastest, most practical tool for confirming whether a stone is a real diamond — and after more than two decades buying, setting, and verifying stones, it is the device I reach for first. But it has clear limits that most buyers do not understand, and trusting the wrong type of tester can cost you. Here is exactly how it works and where it fails. For the physical properties that make this test possible, see our explainer on whether diamonds can get scratched.
How Does a Diamond Tester Actually Work?
A diamond tester uses a heated metal probe tip that touches the stone and measures how quickly heat transfers away from that point. Diamond has exceptionally high thermal conductivity — it pulls heat away faster than almost any other material — so the tester registers that rapid transfer and signals a real diamond, usually with a beep or a bar reading. Simulants like glass and cubic zirconia conduct heat slowly, so they read as fake.
The science is straightforward: every material moves heat at a characteristic rate, and diamond’s rate is far higher than its common imitations. According to how thermal conductivity works, diamond is one of the best heat conductors known, which is precisely why a thermal probe can identify it so reliably. The probe heats, makes contact, and the speed of heat loss tells the device what it is touching.
What Are the Main Features of a Diamond Tester?
A typical diamond tester is a portable, battery-powered handheld device (usually one 9V battery) with a sharp needle-like probe, an audible beep that changes by material, and a calibrated or color-coded scale. It is designed for both experts and novices: contact the stone, and the sound plus the reading tells you diamond, simulant, or metal. Portability lets jewelers test on the go, and the low price puts it within reach of regular buyers.
The audible feedback is the most useful feature in practice. The probe emits a different sound for diamond, for other gemstones, and for metal, so even a beginner can interpret the result quickly. A continuous beep on metal, for example, simply means the probe touched the setting rather than the stone — useful for avoiding false readings during testing.
Is a Diamond Tester Accurate?
Yes — diamond testers are highly accurate and reliable for separating diamonds from most simulants like cubic zirconia and glass. The one major exception is moissanite: it shares nearly identical thermal conductivity with diamond, so a heat-only tester can mistake moissanite for a real diamond. This single blind spot is the most important limitation to know before trusting any thermal reading.
Because of the moissanite problem, the accuracy of your result depends entirely on which type of tester you use. A heat-based tester is dependable against cheap fakes but unreliable against moissanite, which has become a common diamond alternative. If there is any chance the stone is moissanite, a thermal reading alone is not enough — you need the electrical test described below. For more on the most common simulant, see what cubic zirconia is made of.
Heat Tester or Electrical Tester — Which Is Better?
For separating diamond from moissanite, the electrical-conductivity tester is essential. Diamond and moissanite conduct heat almost identically, so a thermal tester cannot tell them apart — but they differ in electrical conductivity, which an electrical (or combination) tester measures. The best modern devices test both thermal and electrical conductivity, giving a reliable result against every common imitation including moissanite.
If you only ever test against cubic zirconia and glass, a heat tester is fine. But moissanite is widespread enough now that I recommend a dual thermal-plus-electrical tester for anyone making real buying decisions. Moissanite looks and tests close enough to diamond that the electrical step is the only consistent way to separate the two. For how the two stones differ overall, see the main difference between cubic zirconia and diamond.
Do Diamond Testers Work on Other Gemstones?
Diamond testers can read other gemstones too, since the underlying technology measures conductivity in any stone. They work on emeralds, sapphires, and rubies, telling you whether the stone is genuine. Ruby is the trickiest — darker rubies can be hard to distinguish from garnet — but the tester still detects them. The device indirectly registers metal as well, beeping continuously when the probe touches a metal setting rather than a stone.
That said, a diamond tester is not a substitute for proper gem identification. It is best at confirming diamond versus simulant; for identifying or grading colored stones, a gemologist’s tools are more appropriate. On metals, it can only tell you something is metallic — not whether it is gold or silver. For a specific metal like silver, a chemical test such as the nitric acid test is the right tool.
Do Diamond Testers Work on Lab-Grown Diamonds?
No — and this is the most misunderstood point. A diamond tester cannot distinguish a lab-grown diamond from a mined one, because both are pure carbon with identical chemical, physical, and thermal properties. A lab-grown diamond will register as a real diamond on any tester, because it is a real diamond. The tester confirms the material is diamond; it says nothing about origin.
To separate lab-grown from mined diamonds you need advanced equipment and, ideally, a gemological certificate — a handheld tester will never do it. This is actually a point in favor of lab-grown stones: they pass every authenticity test a mined diamond passes. If you want origin and quality confirmed, rely on certification rather than a probe, as we cover in whether Costco diamonds are GIA certified.
The Bottom Line
A diamond tester works by measuring how fast heat — and in better models, electricity — moves through a stone, and diamond’s exceptional conductivity makes it stand out from cheap simulants instantly. It is accurate, affordable, and easy enough for a beginner, with two limits you must respect: a heat-only tester cannot separate diamond from moissanite, and no tester can separate lab-grown from mined diamond. Buy a combination thermal-and-electrical tester to defeat the moissanite blind spot, and use certification — not a probe — when origin or grade matters. Used within those limits, a diamond tester is the most useful authenticity tool a buyer can own.
Frequently Asked Questions
Can a diamond tester be fooled?
Yes — by moissanite, if you use a heat-only tester. Moissanite has thermal conductivity so close to diamond that a thermal probe often reads it as genuine. The fix is a tester that also measures electrical conductivity, since diamond and moissanite differ electrically. Heat-only testers are not fooled by cubic zirconia or glass, which conduct heat slowly, but moissanite is their consistent weakness. Always use a dual-function tester if moissanite is a possibility.
Does a diamond tester work on gold or silver?
Only indirectly, and not for identifying the specific metal. When the probe touches any metal, the tester beeps continuously to indicate a conductive metallic surface — but it cannot tell you whether that metal is gold, silver, or something else. To identify a specific metal you need a dedicated test, such as a nitric acid test for silver or an acid scratch test for gold karat. The diamond tester is a stone tester first.
Will a diamond tester work on lab-grown diamonds?
It will read them as real diamonds, because lab-grown diamonds are chemically and physically identical to mined ones — they are genuine diamond, just grown in a lab. No handheld tester can distinguish lab-grown from mined; that requires specialized gemological equipment and certification. If your goal is to confirm a stone is diamond, the tester works perfectly on lab-grown stones. If your goal is to confirm origin, you need a grading report instead.
Are cheap diamond testers reliable?
Inexpensive heat-based testers are reliable for separating diamond from common fakes like cubic zirconia and glass, which is enough for many buyers. Their weakness is moissanite, which they can misread as diamond. If you only need to screen out obvious simulants, a budget thermal tester is fine. If accuracy against moissanite matters — and increasingly it does — spend a little more on a combination thermal-and-electrical model, which is far more dependable across all common imitations.
Stephanie is a jewelry lover when she was a teenager. Her major was fashion design when she was in college. She is a jewelry designer at SOQ Jewelry and other design companies. Now she is also a writer for our website. She writes a lot of designs&brands posts with very actionable tips.