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Handheld Diamond Testers
Handheld Diamond Testers

An in-depth look at handheld diamond testers and why they are sometimes unreliable

Karly B. (GIA Gemologist) avatar
Written by Karly B. (GIA Gemologist)
Updated over a week ago

Handheld Diamond Testers - Do They Work?

Since most people (and jewelers) are not graduate gemologists and can’t tell the difference between a natural diamond, lab grown diamond, moissanite or even CZ with the naked eye, but still want to check and make sure their diamond is indeed a diamond, a lot of jewelers will use a handheld diamond tester to help. It’s a great little tool to get an answer quickly, don’t get me wrong, but are they accurate? Short answer, sometimes depending on the material being tested and the person doing the testing. Read on dear friends for the long answer (it’s good, I promise).

What exactly is a handheld diamond tester and how does it work?

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The most commonly used, and easily accessible (they are about 15 bucks on Amazon), handheld diamond tester or ‘pen tester’ for short is a small, portable instrument that measures a diamond’s thermal conductivity. In other words, how a diamond or other gemstone handles heat. This type of tester uses a small metal point that looks like a needle which, when put on the surface of the stone, will measure how quickly heat travels through it. When it works properly (we’ll get to that later), it can separate a diamond from a simulant with the exception of moissanite. Why moissanite? Well, it conducts heat too, very similarly to a diamond so they often give off a false reading. There are moissanite testers on the market as well, that measure the electrical conductivity of a stone, but they are less common so for this article, I'll be talking about the thermal conductive testers.

Does a handheld diamond tester work?

Sure…sometimes. It really depends on how it’s being used. They are temperamental little buggers and any little mishap can make them go haywire and give a false moissanite reading to a diamond (natural, but especially lab grown). If you are going to use a diamond tester, or take your stone to a local store that uses them, here are a few things to look out for.

  1. Make sure it has time to warm up before touching it to the stone. There is usually an indicator light and beep to let you know it’s ready for testing. Don’t be hasty and use it before it’s ready or you’ll likely get a false reading. Can’t wait 30 seconds, try naming all the different diamond clarity grades in your head while you wait!

  2. Don’t let it touch any metal! If the metal needle ever so slightly touches any metal on the surrounding setting it’s all over with. Ok, I’m being a little dramatic here, but it will definitely give you a false reading and you’ll need to start over. My suggestion if this happens…turn the tester off and back on again to reset - it’s not going to hurt giving it a good reboot.

  3. Be sure to wait for at least 30 seconds between tests. I’m telling you, these things are temperamental. If you test a stone and then immediately test another one it’s likely you’ll get a false reading.

  4. Make sure the person using the tester knows the above guidelines. This one is important. Oftentimes when you’re at a jewelry store having your diamond tested, the person behind the counter has little to no gemology/diamond training except for in sales (that’s why they are using the tester in the first place). To combat this, ask if they have a graduate gemologist on staff or chat with us to find one local to you).

Can Diamond Simulants Test as Diamond?

Yep, they sure can. But first, what even is a diamond simulant? A diamond simulant is any material that looks similar to a diamond but doesn’t have the same chemical, physical or optical make-up. They are just look-a-likes to the real thing. CZ, white sapphire, zircon and moissanite (among others) are all diamond simulants. The first three will rarely test as diamond on a handheld tester because they just don’t have the same heat conductivity (sweet, that’s easy), but a moissanite can often test as a diamond (and vice versa) so other distinguishing tests are needed to confirm the true identity of your stone.

What other tests can be done to determine between a moissanite and a diamond?

There are a few quick and easy ways to tell the difference between the two!

  • Grab a jeweler's loupe or even a high-powered magnifying glass and look for the laser inscription. Lab-grown diamonds all have them whereas most moissanite will not. Depending on the lab that graded the diamond, IGI, GCAL & GIA being the big three, you will either see the word ‘lab grown (or created) diamond’ or LG followed by the report number laser inscribed on the girdle. You can then look up the grading report number on the respective grading lab's website to confirm the diamond.

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  • The most accurate and surefire way to distinguish between a diamond (natural or lab grown) and a mossianite is to check for doubling. A moissanite is doubly refractive, a diamond is singly refractive. This means, that when you look through the bezel facet of a moissanite, you will see the facets reflecting twice versus once with a diamond. (The images below will help explain.) This is very easy for someone who is trained in diamonds and knows what to look for, but remember, most people you meet at the counter of a local store are not going to have this type of training hence why they reach for the handheld diamond tester first.

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  • If all else fails, seek out a graduate gemologist. They will be able to tell you 100% without a doubt whether your stone is a diamond or moissanite. If you need help finding a local GG, reach out to our gemologists via chat.

My Lab Grown Diamond is Testing as Moissanite. What’s the Deal?

We already know that lab-grown diamonds are real diamonds, just grown differently. They have the same chemical, physical and optical properties as their natural counterparts…so why do they sometimes test as moissanite? They have a range of thermal conductivity, depending on the density, just like moissanite. This is especially true for HPHT grown diamonds. (You can check your report paperwork to see if you have a CVD or HPHT grown diamond).

Natural blue diamonds will test as moissanite on a handheld tester too because of the presence of boron (what makes it blue) in the chemical makeup. Oftentimes, growers will add a touch of boron to their lab-grown diamonds to counteract the nitrogen that causes a yellow tint. This is usually not visible to the naked eye in a finished stone, but there is enough present at the molecular level to cause the diamond to react like a moissanite on a diamond tester.

So at the End of the Day, I shouldn't be Worried if My Diamond Tests as Moissanite on a Handheld Tester?

Nope, but of course, we want you to be confident in the diamond that you purchased. As always you are covered under our Rare Carat Guarantee to receive exactly what you ordered. If you still have doubts, we can assist in verifying the diamond with a 3rd party, reputable lab or possibly an appraiser near you.

I hope this helps to ease your mind about handheld diamond testers and why they aren’t always the most accurate. If you ever have any questions, please feel free to reach out to a gemologist via chat in the bottom right corner or email us at

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