Do Lab Grown Diamonds Pass a Diamond Tester?

How a thermal tester reads a stone

A handheld thermal tester does not detect whether a stone is real in any general sense. It measures how quickly the stone pulls heat away from a warmed probe tip. The tip touches the stone, the tester watches how fast the tip cools, and a light or a display reports the result.

Diamond is one of the best heat conductors of any material at room temperature. Cubic zirconia and glass conduct heat poorly by comparison, so the probe tip stays warm and the tester reads “not diamond”. That contrast is what the thermal test was built to catch, and it does that job well.

Because the test measures a property of the material, it says nothing about where the material came from. It also says nothing about color, clarity, cut or weight. A tester answers one question, whether the material conducts heat like diamond, and stops there.

Why lab-grown diamonds pass

A lab-grown diamond is not an imitation and it is not a lookalike. It is diamond, grown from carbon in a controlled setting instead of formed in the earth. Its atoms sit in the same cubic crystal lattice as a mined stone, and that lattice is what carries heat so quickly.

Same material, same lattice, same thermal conductivity. The probe tip loses heat at the same rate, and the tester reads diamond.

The worry behind the question usually comes from the word lab. A stone made in a lab sounds like a stone made to imitate one, and testers exist to expose imitations. It is the other way around.

An imitation is a different material chosen to look like diamond, such as cubic zirconia, glass or moissanite. A lab-grown diamond is the material itself, so a tester has nothing to expose. The word describes how the stone was made, and the tester never asks how.

That is why the answer does not change with the wording. Do diamond testers work on lab grown diamonds? Yes, exactly as they do on mined ones. Will a lab-grown stone pass the diamond test?

Yes. Does a lab-grown diamond test as real? It tests as diamond, which is what it is. Every version of the question has one answer on a working thermal tester.

The same holds for the other properties a shopper might check with the eye or a loupe. Hardness, brilliance, refractive behavior and density belong to the material, not to the place it formed. Nothing is left over for a conductivity probe to flag.

Anyone weighing a lab-grown diamond can start with our lab-grown diamonds page, where they are listed separately from mined stones. Being lab-grown does not change how a tester reads a stone.

Does moissanite pass a diamond tester?

Moissanite is silicon carbide, a different material with its own crystal structure. It is not diamond, and it is not a lab-grown diamond. It does share one relevant trait with diamond, which is that it conducts heat well. The probe tip loses heat quickly, and a thermal-only tester reads the stone as diamond.

So on a thermal-only tester, moissanite passes when it should not. The tester has not made an error in its own terms. It measured heat flow correctly.

The mistake sits in the question, because heat flow alone cannot separate two materials that both conduct heat well. Many simple pen-style testers work this way.

Combination testers were built to close that gap. A combination tester runs the thermal test and adds an electrical conductivity test. Moissanite conducts electricity where an ordinary diamond does not, so the two readings together point to moissanite instead of diamond. Some testers show a separate moissanite indicator for this reason.

When someone types “does moissanite pass diamond tester” into a search box, the useful answer is that it depends on which tester is in the hand. Here is how the readings split:

• Start with a lab-grown diamond. A thermal-only tester reads diamond, and so does a combination tester.
• Next is a mined diamond. It gives the same result on both types of tester.
• Last is moissanite. A thermal-only tester reads diamond in error, and a combination tester flags it as moissanite.

Other checks that separate moissanite from diamond

A tester should not be the only check on a stone that might be moissanite. Moissanite is doubly refractive, meaning light entering it splits into two paths. Look through the top of the stone with magnification and the back facet edges often appear doubled.

Diamond does not do this. Moissanite also tends to throw more colored flashes than diamond does.

None of these checks needs special equipment beyond a loupe, but they take practice to read. Our guide on how to tell if a diamond is real covers the at-home checks and what each one can and cannot prove.

What a tester cannot tell you

The most common surprise runs the other way from the worry. People wonder whether a lab-grown diamond will fail a tester. It will not.

What a tester cannot do is separate a lab-grown diamond from a mined one. Both are diamond, both conduct heat the same way, and both read as diamond. Nobody should expect a handheld tester to settle where a stone came from.

Separating origin takes laboratory instruments. Spectroscopy and photoluminescence read the faint differences left by how each stone grew, and grading laboratories use them. A pocket tester has no access to any of that.

A tester also cannot grade. Color, clarity, cut and weight are separate judgments, and our diamond buying guide explains how each one works.

Even on the one question it can answer, a tester can be wrong for ordinary reasons. Here are the usual causes:

• Start with a dirty stone or probe tip. Oil and residue slow the heat transfer at the contact point.
• Next is a tester that has not warmed up, or one with a weak battery. Its readings drift and repeat badly.
• Then comes a very small stone. A standard probe tip can be larger than the facet it needs to touch.
• Last is a probe touching the metal of the setting. Metal conducts heat well, so the tester reads the setting instead of the stone.

One narrow exception concerns color. A rare blue diamond, mined or lab-grown, gets its color from boron, and boron changes how the stone conducts electricity. On a combination tester the electrical reading can differ from a colorless diamond’s.

An unusual result on a blue stone is a reason to ask for a laboratory opinion, not a reason for alarm. Treat a single reading as one clue, and repeat it on more than one facet before relying on it.

How to confirm where a diamond came from

If origin matters to you, whether for insurance, for resale or for your own peace of mind, rely on paperwork and not on a tester. A grading report from a gemological laboratory states whether a stone is laboratory-grown or mined, alongside its grades. A laser inscription on the girdle, readable under magnification, can carry a report number and, on a laboratory-grown stone, a note saying so. Not every stone carries an inscription.

With no report and no inscription, the route is a gemologist with laboratory screening equipment. A handheld tester will confirm only that the stone is diamond. That is still useful.

It rules out cubic zirconia and glass, and a combination tester rules out moissanite as well. It does not say whether the diamond is lab-grown or natural.

If you are choosing between the two, the useful comparison is not how they test. They test the same. It is how each one is graded and described when it is sold. Mined and lab-grown stones sit on their own pages here, so you can start with lab-grown diamonds and read the listing for each stone. When you buy lab-grown loose diamonds from us, each stone's page says it is lab-grown.

Bringing a piece in to be looked at

Inspection is free on pieces bought from us. Our showroom is at 1905 Scenic Hwy N Ste 230, Snellville, GA 30078. Bring the piece, and bring any grading report that came with it, since that document is the one that states origin. A tester reading adds one more clue to it and does not replace it.