Does Moissanite Pass a Diamond Tester?

It surprises a lot of people: how can a stone that is not a diamond "pass" a diamond tester? The answer comes down to what those little pen testers actually measure. A basic tester checks heat flow, and moissanite happens to move heat almost exactly like a diamond, so it reads as one. A better tester also checks electricity, where moissanite gives itself away. Understand that single distinction and you will never be fooled again. This guide explains the science, the scam it enables, and the reliable ways to tell moissanite from a diamond at home.

Moissanite is one of the most popular diamond alternatives in the world, and it is April's birthstone; see the stone in our April birthstone guide and browse our moissanite engagement rings as you read. First, the two-sentence version.

The Short Answer

Two kinds of tester, two different results.

Yes on a thermal tester, no on a dual tester

Whether moissanite "passes" hinges entirely on which tester is used. A basic, inexpensive pen-style tester measures heat only, and moissanite passes it as diamond. A modern dual tester adds an electrical check, and it correctly flags moissanite. So the honest answer is: yes on a cheap tester, no on a good one. Passing a heat-only tester never proves a stone is a diamond; it only proves the stone conducts heat like one. Everything below explains why.

How Diamond Testers Work

What the pen is actually measuring.

A standard "diamond tester" does not analyze what a stone is made of. It measures how the stone moves energy, and there are two very different properties in play.

How thermal (heat) testers work

The common, low-cost pen tester measures thermal conductivity — how quickly heat passes through the stone. You warm a tiny probe tip, touch it to the stone, and the device times how fast the heat drains away. Diamond is one of the best heat conductors on Earth, so a thermal tester treats "heat drains fast" as its "diamond" signal. The trouble is that this signal is about heat flow, not identity.

Electrical and dual testers

A second property is electrical conductivity — whether the stone carries a small electrical current. This is where diamond and moissanite part ways. Diamond is an electrical insulator; it blocks current. Moissanite conducts. A dual tester (sometimes sold as a "moissanite tester") runs the heat test first, then applies a tiny voltage and measures resistance, so it can separate the two stones that a heat-only tester lumps together. As the International Gem Society notes, telling moissanite from diamond reliably calls for a "moissanite tester" that measures electrical conductivity, plus a loupe for a close look.

Why Moissanite Passes a Thermal Tester

Near-identical heat conductivity.

Why moissanite passes

Moissanite is silicon carbide, and like diamond it is an outstanding heat conductor — its thermal conductivity sits close enough to diamond's that a thermal-only tester cannot separate them. When the warm probe touches a moissanite, heat flows through almost exactly the way it would in a diamond, and the tester lights up "diamond." The device is not broken or defective; it is doing precisely what it was built to do, which is measure heat flow. Moissanite genuinely matches diamond on that one property. Because most cheap pocket testers are thermal-only, this is why the "moissanite passes!" claim is everywhere. It is true, and also incomplete: passing means "conducts heat like a diamond," not "is a diamond."

Why a Dual Tester Catches It

The electricity giveaway.

Here is the catch that exposes moissanite: unlike diamond, moissanite conducts electricity. A real diamond passes the heat test and then shows no electrical conductivity. Moissanite passes the heat test but then registers a current, so a dual tester flags it as moissanite, not diamond. That one extra check is all it takes to tell them apart reliably, which is why a dual tester is the tool you actually want. The table below shows how each type of tester responds to a moissanite.

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Moissanite vs Diamond: The Real Differences

Same sparkle, different physics.

A tester reading is only one clue. Moissanite and diamond differ in several measurable ways, and those differences are what a jeweler, a loupe, or a dual tester keys on. According to the International Gem Society's moissanite profile, moissanite is crystalline silicon carbide with a higher refractive index and far greater dispersion than diamond, which is exactly why it throws more colored fire.

Six Ways to Tell Moissanite From a Diamond

Confidence without a lab.

Other ways to tell moissanite from a diamond

You do not need a laboratory to make a confident call. Working from most to least reliable:

1. Use a dual tester. The thermal-plus-electrical check is the most reliable at-home method, and it is what most modern jewelers reach for first.
2. Look for doubling under a loupe. Peer straight through the top facet at the back edges: moissanite is doubly refractive, so those facet junctions look doubled or slightly blurred. A diamond is singly refractive, so its edges stay crisp and single. This is the classic gemologist's giveaway.
3. Watch the fire. Moissanite's dispersion is roughly 2.4 times diamond's, so it throws noticeably more rainbow-colored flashes. A diamond's sparkle is whiter and more restrained; a big moissanite can look "disco-ball" colorful.
4. Check for certification. A diamond (mined or lab-grown) comes with a GIA or IGI report and often a laser girdle inscription. Moissanite will not carry a diamond certificate.
5. Consider the price. If a large, "flawless" diamond is remarkably cheap, treat it as a red flag — it may well be moissanite.
6. Ask a jeweler. A professional can confirm identity in seconds, and an independent gem lab is definitive.

Will a Jeweler Know?

What fools a pen does not fool a professional.

Will a jeweler know it is moissanite?

Yes. A trained jeweler or gemologist can identify moissanite quickly and reliably. Beyond a dual tester, the diagnostic tells to a professional are the birefringence (that visible facet doubling), the extra fire, and moissanite's characteristic inclusions and surface features under magnification. In other words, the very cheap thermal pen that "proves" a moissanite is a diamond is the one tool that cannot actually make the call — while any competent jeweler can. If you want certainty on a stone you already own, an independent gemologist or a gem lab settles it for good.

Lab-Grown Diamonds and CZ Behave Differently

An important contrast — not every "alternative" acts the same.

Lab-grown diamonds do pass — because they are diamond

Do not confuse moissanite with a lab-grown diamond. A lab-grown diamond passes both thermal and dual testers as "diamond," because it genuinely is a diamond: same carbon, same heat conductivity, same electrical-insulating behavior as a mined stone. Only its origin differs. If a stone passes a dual tester as diamond, it is a real diamond — natural or lab-grown — not a simulant. For the full explanation, see our guide on whether lab-grown diamonds are real diamonds and our is moissanite a lab-grown diamond? explainer. Browse the real thing in our lab-grown diamond engagement rings.

Cubic zirconia fails even a basic tester

Cubic zirconia is the opposite case. Unlike moissanite, CZ has low thermal conductivity, so even a basic thermal pen correctly rejects it as "not diamond." That is the tidy way to remember it: moissanite mimics diamond's heat conductivity and slips past a cheap tester, while CZ does not and gets caught immediately. We break the two down in our moissanite vs cubic zirconia guide, and compare the top two picks in moissanite vs lab diamond.

What This Means for Buyers

Moissanite is still wonderful — buy it honestly.

Because a basic tester can be fooled, it can also be misused. A dishonest seller could wave a cheap thermal pen over a moissanite, show you the "diamond" light, and charge diamond prices — a genuine scam that relies on buyers not knowing the difference. Now you do. The real protections are simple: insist on a reputable grading report, buy from sellers who clearly disclose exactly what the stone is, and get an independent gemologist's opinion when in doubt.

None of this is a knock on moissanite. It is a stunning, hugely popular gem, second only to diamond in hardness at 9.25 on the Mohs scale, more fiery than diamond, and a fraction of the price. For many buyers, that blend of durability, sparkle, and value makes it the smarter pick — the only thing that matters is honesty. Learn what it stands for in our moissanite meaning guide, plan your budget with how much to spend on an engagement ring, and see another clever simulant in white topaz vs diamond. Explore the collection in moissanite jewelry, or compare it beside diamond jewelry.

Moissanite & Diamond Tester FAQ