A basic diamond tester measures how quickly heat moves through a gemstone, and its reading is only a screening signal. Natural diamonds and lab-grown diamonds usually register as diamond, while moissanite can also pass on some thermal pens. That overlap helps explain the curiosity behind teenagers testing their teachers rings.

A result cannot prove that a stone is natural, lab-grown, or unquestionably authentic. Use the pen as a starting point, then look at the device’s limits and the right follow-up for the stone in front of you.

What is a diamond tester?

When someone says “diamond tester,” they usually mean a handheld diamond checker or diamond detector with a probe that measures a gemstone’s thermal conductivity. In other words, it checks how quickly heat moves away from the tip through the stone.

How do diamond testers work?

Basic thermal pens screen for materials with diamond-like heat transfer. Combination testers also assess electrical conductivity, which can help separate some stones that overlap on a thermal reading. The exact lights, bars, or tones vary by model, so follow your device instructions before interpreting a result.

Do Diamond testers really work?

Diamond testers work well as screening tools when used correctly, especially for separating many lower-conductivity simulants such as CZ from diamond. But are diamond testers accurate enough to stand alone? No. Accuracy depends on the tester type, setup, stone, environment, and technique. Moissanite can overlap with diamond on a basic thermal reading, and a pen cannot identify natural versus lab-grown origin.

How to use a diamond tester correctly

Every diamond tester pen is different, so start with the manufacturer’s instructions for warm-up, calibration, and its result scale.

  1. Read the device instructions for its setup and limits.
  2. Clean and dry the stone so residue or moisture doesn’t affect contact.
  3. Allow the tester to warm up or calibrate as directed.
  4. Place the probe straight down on a flat facet.
  5. Keep the tip off the metal setting.
  6. Repeat the reading on another suitable facet.

Some devices beep, light a scale, or show a bar. Treat that signal only according to that model’s instructions because one pen’s “diamond” reading may not mean the same thing on another.

Why diamond testers give false results

False readings usually come from a dirty or wet stone, room or stone temperature, a low battery, skipped calibration, a worn or damaged probe, contact with metal, very small facets, or use outside the tester’s stated scope. Take multiple readings after correcting setup issues. If results stay inconsistent, stop and use a trained gemologist or laboratory.

What should you check after a tester says “diamond”?

A “diamond” reading should lead to the grading report, not a final verdict about the stone.

When a grading report is available, compare its number, carat, cut, color, clarity, and measurements with the stone in front of you. Do Amore’s diamond education treats that report as an identity document, not just a quality score. The tester narrows the material. The report and qualified screening address identity and origin.

Can fake diamonds pass diamond tester?

Will moissanite pass a diamond tester? On some basic thermal pens, yes. Moissanite may register as diamond because this lab-created gemstone also transfers heat very efficiently. That result doesn’t make moissanite a diamond; moissanite engagement rings use a distinct gemstone with unusually high thermal conductivity. Care, residue, and stone quality all matter when evaluating whether moissanite gets cloudy or simply needs a thorough cleaning.

CZ and white sapphire usually do not register as diamond on a basic thermal tester. But a typical result isn’t guaranteed, so confirm an uncertain stone with appropriate gemological testing.

Note the CZ’s (left) lack of depth and “plasticky” reflection vs the Diamond’s (right) multi-tonal shades and crisp reflection?

Typical basic thermal-pen results vary by model, stone, and testing conditions. Use this table as a screening guide, not a guarantee.

Stone Typical basic thermal result What the result does not prove Next step
Natural diamond Usually reads as diamond That the stone is natural or matches a report Check the grading report; use a laboratory if identity matters
Lab-grown diamond Usually reads as diamond Whether the stone is natural or lab-grown Check the report and use origin screening when needed
Moissanite May read as diamond on some pens That the stone is diamond Use a combination tester or trained gemologist
Cubic zirconia Usually does not read as diamond That a negative result identifies CZ Confirm with other gemological tests
White sapphire Usually does not read as diamond That a negative result identifies sapphire Confirm with other gemological tests

Diamonds and Diamond Testers

How do Jewelers tell if diamonds are real?

Trained gemologists identify a stone by combining visual examination with appropriate instruments. They may assess optical properties, thermal and electrical response, and other features that separate diamond from moissanite, CZ, white sapphire, and white topaz. A scratch test isn’t a safe home check. GIA’s guidance recommends a trained gemologist or independent laboratory when identification matters.

Does a real diamond sparkle rainbow?

Yes! Rainbow sparkle in a diamond is called “fire”. There is some misinformation out there that diamonds do not show colored light, however this is false. In fact, a diamond’s ability to show a rainbow of color is one of its most prized features, and one of the reasons it is preferred by many over other white stones like white sapphire or white topaz. Moreover, diamonds show a balanced display of rainbow and white sparkle.

This should be compared against moissanite and CZ’s major colored sparkle, which outshines its white sparkle.

Do lab diamonds pass the diamond tester?

Yes. A lab-grown diamond will usually register as diamond on a basic thermal tester because its physical properties closely match those of a natural diamond. That means a pen cannot answer the origin question. If you’re comparing lab diamond engagement rings or checking a stone you own, use the grading report and appropriate origin-screening tools rather than relying on one thermal reading.

Can a diamond tester tell natural from lab-grown?

No. A basic thermal pen can’t tell whether a diamond formed in the earth or in a laboratory. Some screening instruments check additional properties and may return “pass” or “refer,” but those results must be interpreted within the tool’s stated scope. GIA explains that definitive origin identification can require advanced gemological testing. Some instruments may respond differently to certain HPHT lab diamonds, which is why a refer result is a cue for more testing, not a final identification. If a lab-grown diamond produces a confusing result, our moissanite vs. lab diamond guide explains why the stones still differ even when a tester reading overlaps. Here’s what to check next:

Ask what training and instruments are being used. For a valuable or uncertain stone, consider an independent qualified gemologist or appraiser (for example, a member of NAJA).

Treat visual clues as supporting evidence. A trained gemologist may examine double refraction (“doubling”) and other optical features, but one observation is not always conclusive. If the result affects a purchase, insurance, resale, or dispute, ask for laboratory confirmation.

Check the grading report and look for a microscopic certificate number inscription if the report says one should be present. Many graded diamonds carry a girdle inscription, and GIA lab-grown diamond reports include one, but an absent or matching inscription is not conclusive by itself.

How do you test a lab grown diamond?

Start with the grading report and any readable girdle inscription, then use appropriate screening for origin. Because natural and lab-grown diamonds share many chemical, optical, and physical properties, a basic thermal pen and routine observation cannot reliably separate them. GIA uses advanced instruments and laboratory testing for definitive identification. If you’re weighing lab-grown vs. natural diamonds, choose based on your priorities, not a pen result.

Can a diamond tester tell you which diamond to choose?

No. A tester screens the material, but it cannot compare the tradeoffs that shape a ring purchase.

Do Amore offers responsibly sourced, conflict-free natural and lab-grown diamonds without pushing shoppers toward either option. Compare the grading details, budget, and personal preferences that matter to you, then use the pen result only for what it can show.

What stones will pass a diamond tester?

Natural diamonds and lab-grown diamonds usually pass, and moissanite can also pass on some basic thermal pens. A handheld tester measures heat transfer, not chemical composition or origin, so the reading is one piece of the identification process.

Moissanite (left) vs White Sapphire vs Diamond (right)

Moissanite and Diamond Testers

Can a diamond tester detect moissanite?

Sometimes. A combination tester may indicate moissanite, while a basic thermal tester may read it as diamond. The outcome depends on the model and testing conditions. A “diamond” or “moissanite” signal narrows the possibilities but doesn’t settle identity. Follow the device instructions and use trained gemological examination when needed.

Is there a moissanite tester?

Yes, combination testers can use thermal and electrical conductivity to help separate moissanite from diamond. A trained gemologist may also examine optical features, including double refraction, but no single check should be treated as conclusive when the stone is valuable or uncertain.

How do you test for diamond and moissanite?

Use a tester only as the first screen. Then confirm the result with the stone’s documentation and a trained examination.

  • Test thermal conductivity. A basic pen may group diamond and moissanite together, so the signal narrows the field rather than identifying the stone.
  • Check for a girdle inscription and verify any report number with the issuing laboratory. Many graded diamonds carry inscriptions, but the absence of one does not rule out diamond.
  • Have a trained gemologist evaluate optical features such as double refraction and use appropriate instruments. Don’t rely on a single view through a loupe.

How to test for Moissanite using doubling instructions

How to test for Moissanite using doubling instructions

CZ and Diamond Testers

Can cubic zirconia pass a diamond tester?

Usually, no. Cubic zirconia (CZ) doesn’t transfer heat quickly enough to register as diamond on most basic thermal pens. Still, device setup and stone conditions can affect a result, so a negative reading is not enough to identify the material.

How can you tell a real diamond from a CZ?

Thermal testing can help separate the two, but a trained gemologist combines that result with visual examination and appropriate instruments. Many graded diamonds carry a microscopic girdle inscription, yet the presence or absence of an inscription alone is not proof. Avoid home scratch tests or improvised light tests.

Cubic zirconia has a specific gravity of about 5.6 to 6.0, compared with 3.52 for diamond. That makes CZ more than 1.5 times as dense, so a loose same-size stone can feel noticeably heavier.

White Sapphire and Diamond Testers

Do white sapphires pass a diamond tester?

No. White Sapphire does not have high enough thermal conductivity to pass. And, white sapphire looks quite different than diamond, moissanite, and CZ, so using a pen tester is not common. Whereas the high conductivity gems show rainbow sparkle, white sapphire shows white and silver sparkle. It also tends to have a silvery undertone. So, these are often easier ways to spot sapphire from diamond.

How could you tell a white diamond from a white sapphire?

Whereas diamond will pass a pen test and have rainbow sparkle, white sapphire will not pass and has white and silver sparkle. Sapphire is usually faceted differently than diamond, as well, with rectangular facets on the bottom of the stone. Finally, more nuanced tests include finding the “refractive index” of the gem. Notably, white sapphire’s refractive index also helps to distinguish white sapphire from white topaz!

Moissanite (top) vs White Sapphire (bottom) ovals

Diamond Tester FAQ

Are diamond testers accurate?

Diamond testers can be accurate as screening tools when the correct device is clean, calibrated, and used as directed. Their result is not a guarantee of identity or origin, especially when moissanite or lab-grown diamond is possible.

Can you test a diamond in a mounted ring?

Often, yes, if the tester tip can touch a suitable flat facet without contacting metal. Settings can make access harder and metal contact can distort some readings, so repeat the test and follow the model’s instructions.

Can a diamond tester test very small stones?

Some devices have minimum-stone-size or probe-position requirements. Small facets are harder to contact cleanly, so check the tester’s stated limits and use a gemologist when the stone is too small for a reliable reading.

Can a diamond tester damage a stone?

A correctly used electronic probe should not scratch a diamond, but pressing too hard, slipping into the setting, or using destructive home tests can damage the stone or metal. Use light contact and never substitute a scratch test.

When should I use a gemologist or laboratory?

Use an independent qualified gemologist or gemological laboratory when the result affects a valuable purchase, insurance, resale, or an identity dispute, or when repeated readings disagree. That’s also the safer next step for natural-versus-lab-grown origin.

Can you answer more of my questions about diamond testers?

Do Amore doesn’t offer appraisal or gem-testing services. If a result affects a valuable purchase, insurance, resale, or a stone-identity dispute, ask an independent qualified gemologist or gemological laboratory for the next step. When you’re choosing a ring, work with a jeweler that explains what was tested, which report supports the stone, and what the result does not prove. You can also read verified customer experiences (find them on Facebook, The Knot and Wedding Wire)!

When a pen result leaves you with more questions than answers, choose a ring with clear documentation and guidance behind it. Do Amore offers responsibly sourced, conflict-free natural and lab-grown diamonds, and every purchase gives one person access to clean water. Browse Do Amore’s engagement rings to compare both options with a clearer sense of what the tester can and cannot tell you.