How to Tell if a Diamond Is Real, Lab-Grown, or Moissanite: A Gemologist's Guide to Testers & Screeners

Separating a natural diamond from a lab-grown diamond, moissanite, or cubic zirconia is one of the most common — and most misunderstood — challenges in the gem trade. The confusion exists because these materials share overlapping physical properties, and no single handheld device solves every case. This guide explains, in plain terms, how diamond testers and diamond screeners actually work, where each succeeds or fails, and how to build a reliable, layered workflow you can trust.

Quick answer

A standard thermal-conductivity diamond tester separates diamonds from most simulants (like cubic zirconia), but it will read moissanite as “diamond” because moissanite conducts heat as efficiently as diamond. To catch moissanite, use a tester that also measures electrical conductivity, since moissanite is a semiconductor and conducts electricity while most diamonds do not. Critically, no handheld tester separates natural from lab-grown diamonds — that requires a diamond screener that gives a “pass” or “refer” result, followed by advanced lab instruments for conclusive identification.

Tester vs. screener: two different jobs

The words “tester” and “screener” are often used interchangeably, but they do fundamentally different jobs:

  • Testing (detection) positively identifies whether a stone is a diamond, moissanite, or a simulant — typically by measuring thermal and electrical conductivity.
  • Screening separates natural diamonds from lab-grown diamonds. Because a diamond’s growth origin cannot be judged by eye, screening equipment is the practical first line for sorting mined stones from CVD and HPHT synthetics.

You can browse both instrument families in the Diamond & Gem Testers category, with dedicated origin-sorting devices grouped under Synthetic Diamond Screeners.

How diamond testers work

Diamond testers determine identity by measuring how a stone conducts energy. There are two core technologies:

1. Thermal conductivity testers. A small metal probe touches the stone and measures how quickly heat dissipates. Diamond is an exceptional heat conductor, so it sheds heat faster than most imitations such as cubic zirconia or glass.

2. Electrical conductivity testers. These measure whether a stone conducts electricity. Most diamonds are electrical insulators, whereas moissanite conducts a small current — the key property that separates the two.

The moissanite problem (and why it matters)

Moissanite is the trickiest simulant. Its thermal conductivity is similar to — and often higher than — diamond, so a basic thermal-only tester will pass it as diamond. The reliable fix is a dual-test instrument that checks thermal and electrical conductivity in a single pass. On the electrical test, moissanite reveals itself.

Note: roughly 2% of natural diamonds are “Type II” (boron-bearing) and are electrically conductive, so they can falsely flag as moissanite on some testers. Always corroborate an ambiguous reading with a second method.

Comparison: what each method can and can’t do

Method Diamond vs. CZ / simulants Diamond vs. moissanite Natural vs. lab-grown
Thermal only Reliable Fails (moissanite passes as diamond) Cannot separate
Thermal + electrical Reliable Detects most moissanite Cannot separate
Synthetic diamond screener (UV transmittance) N/A N/A Sorts “pass” (natural) vs. “refer”
Lab spectroscopy / deep-UV imaging Reliable Reliable Conclusive

Why testers can’t separate natural from lab-grown diamonds

Lab-grown diamonds (CVD and HPHT) are chemically and physically diamond, so they share the same thermal and electrical behavior as natural diamonds and read simply as “diamond.” This is why a conductivity tester alone can never confirm origin.

Origin sorting instead relies on a synthetic diamond screener. These portable devices measure a colorless stone’s transparency to shortwave ultraviolet (SWUV) light to sort diamonds into types. Type Ia diamonds absorb SWUV and are natural; Type IIa diamonds transmit SWUV and are commonly — though not always — laboratory-grown. Good screeners handle melee as small as roughly 0.02–0.03 ct and return a result in about two seconds.

The “pass” and “refer” principle

A key point every jeweler should understand: consumer-market screeners do not “confirm lab-grown.” They give a pass (the stone is natural) or a refer (the stone needs further testing). A “refer” result may be natural or lab-grown — it simply means the fast screen could not clear it. There is no cost-effective, rapid tool that definitively identifies every laboratory-grown diamond, which is why a “refer” stone is escalated to a combination of gemological techniques or a laboratory.

A practical, layered testing workflow

  1. Screen for material with a dual thermal + electrical tester to sort diamond, moissanite, and simulants.
  2. Screen for origin on colorless diamonds using a synthetic diamond screener to get a pass/refer result.
  3. Observe ambiguous stones with a loupe, microscope, or polariscope for strain patterns and inclusions.
  4. Escalate any “refer” stone to spectroscopy / deep-UV imaging or a reputable laboratory for a conclusive origin determination.

For steps 1 and 2, the instruments you need live in the Diamond & Gem Testers and Synthetic Diamond Screeners collections.

Buyer’s checklist: choosing the right instrument

  • What do you actually need to answer? “Is it a diamond?” needs a tester; “Is it natural or lab-grown?” needs a screener.
  • Does it test both thermal and electrical conductivity? Thermal-only devices miss moissanite.
  • Can it handle mounted stones and small melee? Important for repair and estate work.
  • Does it read out clearly (pass/refer, color-coded lights)? Reduces operator error.
  • Is it from a recognised gemological brand? Repeatable accuracy matters more than price.

Frequently asked questions

Do lab-grown diamonds pass a diamond tester?
Yes. Because they are true diamond, they read as “diamond” on standard testers; a handheld conductivity device cannot tell natural from lab-grown.

Will moissanite pass a diamond tester?
On thermal-only testers, usually yes. On a combined thermal + electrical tester, most moissanite is correctly identified.

What is the difference between a diamond tester and a diamond screener?
A tester separates diamond from moissanite and simulants. A screener separates natural diamonds from lab-grown ones by returning a pass/refer result.

Can any handheld tool confirm a diamond is lab-grown?
No. Screeners flag stones for referral; conclusive origin identification requires spectroscopy or deep-UV imaging, done with advanced instruments or at a laboratory.

Conclusion

Accurate identification is a sequence, not a single gadget: test the material, screen the origin, observe, and escalate anything uncertain. Pairing a dual-conductivity tester with a synthetic diamond screener covers the vast majority of day-to-day jewelry and gem work — and it protects your customers and your reputation. Explore the full range in our Diamond & Gem Testers and Synthetic Diamond Screeners collections.

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