Do Lab-Grown Diamonds Pass a Diamond Tester?

Many people become concerned when a diamond tester gives an unexpected or inconclusive result on a lab-grown diamond. This confusion often leads to a common question: Do lab-grown diamonds actually pass diamond testers?

The short answer is yes, lab-grown diamonds do pass diamond testers. However, the complete explanation is more nuanced than a simple confirmation.

Lab-grown diamonds are real diamonds. They share the same chemical composition, crystal structure, and thermal conductivity as natural diamonds. Because most handheld diamond testers rely on thermal or electrical conductivity to identify diamonds, lab-grown diamonds typically register as genuine.

Confusion arises because not all testers are created equal, and some devices are designed primarily to distinguish diamonds from simulants like cubic zirconia, not to differentiate between natural and lab-grown diamonds. In certain cases, especially with older or lower-quality testers, results can appear inconsistent or misleading.

The Simple Answer

Lab-grown diamonds pass standard diamond testers with flying colors.

Why? Because lab-grown diamonds are real diamonds. They’re not imitations, simulants, or “fake” stones. They possess identical chemical composition (pure crystallized carbon), crystal structure (cubic lattice), physical properties (10 on Mohs hardness scale), and optical characteristics (refractive index of 2.42) to mined diamonds.

When you test a lab-grown diamond with a thermal conductivity tester, the most common type used in jewelry stores, it registers as a diamond. Period.

Not all diamond testers work the same way, not all lab-grown diamonds are created equal, and understanding these differences can save you from unnecessary panic (like my customer experienced).

How Diamond Testers Actually Work 

Thermal Conductivity Testers: The Industry Standard

The overwhelming majority of diamond testers you’ll encounter, including the ones used in mall jewelry stores, pawn shops, and online resale platforms, are thermal conductivity testers.

Here’s the science: Diamond conducts heat better than virtually any other material on Earth, about 5 times better than copper. When you touch a diamond tester’s probe to a stone, it sends a tiny pulse of heat into the material and measures how quickly that heat dissipates.

If the heat vanishes instantly? You’ve got a diamond.

If the heat lingers? You’re looking at cubic zirconia, glass, synthetic moissanite, or some other diamond simulant.

Lab-grown diamonds have the same thermal conductivity as mined diamonds, approximately 2,200 W/(m·K) at room temperature. This is why they pass thermal testers flawlessly every single time.

Electrical Conductivity Testers

Some modern diamond testers, particularly higher-end models, also measure electrical conductivity. This is where things get slightly more complicated.

Most natural diamonds are Type Ia diamonds, meaning they contain dispersed nitrogen atoms scattered throughout their crystal structure. These nitrogen impurities formed over millions of years as the diamond crystallized deep within the Earth’s mantle. About 98% of natural diamonds fall into this category.

Type Ia diamonds are electrical insulators they don’t conduct electricity well at all.

However, a small percentage of natural diamonds are Type IIa diamonds, the purest form, essentially nitrogen-free. These are extremely rare in nature (less than 2% of mined diamonds) and were historically prized for their exceptional optical properties. Many famous diamonds, including the Cullinan and Koh-i-Noor, are Type IIa.

Here’s the critical insight: The majority of CVD (Chemical Vapor Deposition) lab-grown diamonds are Type IIa pure carbon with virtually no nitrogen.

Some electrical conductivity testers flag Type IIa diamonds as “potentially lab-grown” simply because they’re so pure. This doesn’t mean the diamond is fake, it means it’s exceptional.

Why Your Lab-Grown Diamond Might Get a “Weird” Result

Scenario 1: Testing Too Fast or on the Wrong Surface

User error accounts for the majority of “failed” diamond tests I’ve witnessed.

Diamond testers require:

• Clean surface contact: Oil, lotion, or dirt on the stone creates a barrier that skews results

• Proper angle: The probe must contact a facet squarely, not at an angle

• Adequate pressure: Too light and the thermal transfer fails; too heavy and you risk damaging the tester

• Avoiding metal: If the probe touches the setting instead of the stone, you’ll get a metal reading

Real-world example: I once watched a customer attempt to test their lab diamond through the plastic bag it came in. The tester beeped as “not diamond” because it was literally testing plastic. Once we removed the stone from the bag, it tested perfectly.

Scenario 2: Testing Type IIa CVD Diamonds with Nitrogen-Specific Testers

Some electrical conductivity testers are specifically calibrated to detect nitrogen presence as a proxy for “natural diamond.” This made sense historically when lab-grown diamonds were rare and often nitrogen-rich (particularly early HPHT diamonds).

Today, many CVD lab-grown diamonds are purer than 98% of natural diamonds. When tested on nitrogen-detection devices, they may show “no nitrogen detected” which the device interprets as “not a typical diamond.”

The irony? Your lab-grown diamond is failing the test because it’s too perfect.

Scenario 3: The Metal Inclusion Wild Card

Some HPHT-grown diamonds contain tiny metallic flux inclusions, microscopic bits of iron, nickel, or cobalt from the growth process. These inclusions are so small they’re usually only visible under magnification.

In rare cases, if a diamond tester probe happens to contact directly over a metallic inclusion (which is statistically unlikely), the tester might register the metal and give a confusing mixed reading.

In practice, I’ve personally tested hundreds of HPHT diamonds with visible metallic inclusions, and I can count on one hand the times the tester actually detected them. The inclusions are typically buried too deep in the stone to interfere with surface testing.

Scenario 4: Battery Issues and Calibration Drift

Here’s something jewelers don’t always admit: Diamond testers require regular calibration and fresh batteries to function accurately.

A tester with a dying battery may give false negatives or inconclusive results on perfectly real diamonds, both lab-grown and natural. Similarly, testers that haven’t been calibrated according to manufacturer specifications can produce unreliable readings.

What Diamond Testers CAN’T Tell You

They Can’t Distinguish Lab-Grown from Natural

Standard diamond testers, thermal or electrical conductivity, cannot tell you whether a diamond was grown in a laboratory or mined from the Earth. Both will test as diamonds because both are diamonds.

Think of it this way: If I gave you two ice cubes, one made from glacial water and one from your refrigerator, a thermometer can’t tell you which is which. Ice is ice, regardless of origin.

They Can’t Determine Quality

A diamond tester tells you that you have a diamond. It says nothing about:

• Cut quality: The most important factor in a diamond’s sparkle

• Color grade: Whether it’s a D (colorless) or a K (noticeable yellow tint)

• Clarity grade: The presence, size, and visibility of inclusions

• Certification authenticity: Whether the stone matches its grading report

I’ve tested I-color SI2 diamonds and D-color VVS1 diamonds on the same device. Both beep “diamond” with equal confidence, despite a 10X price difference in per-carat value.

They Can’t Detect Treatments

Diamond testers can’t tell you if a diamond has been:

• HPHT-treated to improve color

• Laser-drilled to remove dark inclusions

• Fracture-filled to hide cracks

• Clarity is enhanced through various methods

These treatments require gemological laboratory analysis using spectroscopy, microscopy, and other advanced techniques.

How Jewelers Actually Identify Lab-Grown Diamonds

So if diamond testers can’t distinguish lab-grown from natural, how do professional jewelers and gemologists do it?

The Microscopic Inscription

Nearly all certified lab-grown diamonds carry a microscopic laser inscription on the girdle (the diamond’s outer edge). This inscription typically includes:

• The certification number (matching the grading report)

• The laboratory’s name (GIA, IGI, GCAL, etc.)

• The words “LAB GROWN” or “LG”

These inscriptions are invisible to the naked eye and barely visible even under 10X magnification. You need a gemological microscope or specialized loupe to read them clearly.

Real-world application: When authenticating a diamond, I always check for inscriptions first. It’s the fastest, most definitive way to identify lab-grown stones that have been certified.

Important caveat: Not all lab-grown diamonds are inscribed. Uncertified stones, older lab-grown diamonds from before inscription became standard, or stones that have been re-polished, may lack this marking.

The Moissanite vs. Lab Diamond Testing 

One of the most common questions I get: “Will my lab-grown diamond test as moissanite?”

Short answer: No. Modern dual-mode testers clearly distinguish between the two.

Moissanite (silicon carbide, SiC) and diamond (carbon, C) are completely different materials. However, they share one key property: exceptional thermal conductivity. Early thermal-only diamond testers couldn’t tell them apart.

This created panic in the jewelry industry in the late 1990s and early 2000s when gem-quality moissanite became commercially available. Suddenly, jewelers needed to distinguish between three categories:

• Natural diamonds

• Lab-grown diamonds (still rare at the time)

• Moissanite simulants

The solution was dual-mode testers measuring both thermal and electrical conductivity.

Moissanite conducts electricity; diamond (both natural and lab-grown) does not (except rare boron-doped blue diamonds, which are Type IIb).

If you’re using a quality dual-mode tester manufactured in the last decade, your lab-grown diamond will never be confused with moissanite.

Conclusion

Lab-grown diamonds are real diamonds, scientifically, physically, and optically identical to mined ones. They pass diamond testers because they share the same composition, structure, hardness, and brilliance. Misconceptions come from outdated knowledge, not from the stones themselves.

By choosing certified lab-grown diamonds and relying on reputable gemological labs and professionals, buyers can make informed decisions with confidence. Ultimately, lab-grown diamonds are not myths or imitations they are a result of modern science, making genuine diamond ownership more accessible, transparent, and trustworthy than ever before.