There’s lots of talk about diamond testers lately! From those just learning that Moissanite can “pass” as diamond to teenagers testing their teachers rings (OMG), the intrigue seems pretty universal. But are “diamond testers” all they are cracked up to be? And how can you know what you have on your finger? I’m here to let you in on the inside scoop:

What is a diamond tester?

When someone refers to a “diamond tester”, most people mean a handheld “pen” tester that measures a gemstone’s thermal conductivity. In other worse, how a gem handles heat. These testers use a small metal point which, when put on a gem’s surface, measure how quickly heat travels through the stone. People use these to test diamonds because diamonds have super high conductivity.

Do Diamond testers really work?

Diamond testers can be a helpful tool as part of a multi-stage test, but should not be solely relied upon. These little pens are a helpful way to separate out CZ from diamond. But that should be the beginning and end of their use. Why? Because other gems also have high conductivity (such as moissanite) and because the testers themselves notoriously malfunction. Reasons I have seen these testers fail include user error (stones will not test correctly if the point touches metal), low battery, or age/abuse of the pen.

Can fake diamonds pass diamond tester?

Yes, some diamond simulants can pass a diamond tester. For example, moissanite, a man made gem, may pass as a diamond on a standard tester pen. This is because moissanite also has very high thermal conductivity! Other diamond simulants, such as CZ, will not pass the tester. So, it’s important to use multiple tests to confirm your stone!

Note the CZ’s (left) lack of depth and “plasticky” reflection vs the Diamond’s (right) multi-tonal shades and crisp reflection?

Diamonds and Diamond Testers

How do Jewelers tell if diamonds are real?

The major tests jewelers use to tell real diamonds from other white stones are confirming they are singly refractive, have 10/10 hardness, and super high conductivity. Moissanite is doubly refractive under magnification, which easily separates it from diamond. CZ, white sapphire, and white topaz do not have high conductivity using a pen tester. Finally, lesser known diamond simulants do not have high hardness, which can be seen through magnification.

Does a real diamond sparkle rainbow?

Yes! Rainbow sparkle in a diamond is called “fire”. There is some misinformation out there that diamonds do not show colored light – however this is false. In fact, a diamond’s ability to show a rainbow of color is one of its most prized features, and one of the reasons it is preferred by many over other white stones like white sapphire or white topaz. Moreover, diamonds show a balanced display of rainbow and white sparkle. This should be compared against moissanite and CZ’s major colored sparkle, which outshines its white sparkle.

Do lab diamonds pass the diamond tester?

Typically a lab diamond will test as diamond on a pen tester because they are pure carbon just like mined diamonds. Testing a lab diamond is a little more complicated than testing a natural diamond, however. That is because lab diamonds are grown using differing methods and over differing courses of time. This results in different crystal densities (and thermal conductivity levels). As a result, some lab diamonds (HPHT especially) may test as Moissanite on a diamond tester and require further testing.

My lab diamond tests as Moissanite, what gives!?

Ironically, just like Moissanite can test as diamond, diamond can also test like Moissanite. This is because diamonds have a range of thermal conductivity, depending upon crystal density. This is especially true for HPHT lab diamonds. So, if this happens to you, don’t immediately assume the worst. Again, these pen testers are not foolproof. Here’s what to do:

  1. Make sure the person who tested your stone knows how to tell diamond from moissanite without a pen tester. If your jeweler was not properly trained (or trained at all – there is actually no training required to write appraisals as a retailer) they may not know how to properly identify the stone. So, ask how they intend to test. If in doubt, locate an independent gemologist appraiser (I personally recommend NAJA).
  2. Check for double refraction (“doubling“) under magnification. When looking through the side facets of a Moissanite under magnification, the culet will appear doubled/fuzzy. Diamonds do not show this feature. So, this is an easy way to test for Moissanite. If your jeweler does not know what double refraction is or how to see it, you need a different jeweler.
  3. Look for your diamond’s certificate number under a scope. Certificate inscriptions are microscopic and have just gotten smaller over the years, so a jeweler’s loupe may not show it. Make sure to use a proper microscope to confirm your certificate number. Moissanites are not individually numbered like diamonds, so finding your diamond’s certificate number is an easy way to confirm your stone.

How do you test a lab grown diamond?

Lab diamonds can be separated from natural diamonds by reviewing the diamond’s certificate inscription underneath a scope. Lab diamonds will be marked “lab grown” or “LG”, whereas naturals will not have these prefixes. Gem scientists are also be able to tell lab diamonds from natural diamonds using advanced magnification, fluorescence, and laser techniques.

What kind of diamonds pass diamond tester?

Natural diamonds, lab diamonds, and non-diamond moissanite! This is because a handheld pen “diamond tester” actually only tests the ability of a gem to conduct heat. It does not test for whether or not the stone is, for example, pure carbon (like diamonds are). So, while the pen tester is helpful – it’s only one piece of the puzzle!

Moissanite (left) vs White Sapphire vs Diamond (right)

Moissanite and Diamond Testers

Can a diamond tester detect moissanite?

Some testers show a moissanite result whereas others test moissanite as diamond. It all depends upon the model of the tester. However, because of the varying levels of diamond thermal conductivity (discussed above) a test as “diamond” or “moissanite” on one of these pens only gives so much information. Once a stone tests as diamond or moissanite on a pen tester, confirming magnification tests should be performed via microscope!

Is there a moissanite tester?

There are pen testers that provide a certain result for moissanite, however, moissanite is best confirmed through viewing “doubling” under magnification. Why? Especially with the growing popularity of HPHT diamonds, which may test as moissanite on a pen tester, viewing doubling under magnification is the easiest way to conclusively separate the two. And it’s easy! Just look through the “crown” facets of the stone under a scope.

How do you test for diamond and moissanite?

There are a few options! If you’re using a pen tester, start there to rule out diamond simulants other than moissanite. Then check for certificate or doubling! Here’s how:

  1. Test for high thermal conductivity. Use that pen tester! Does it come up as diamond? You probably have either a diamond or a moissanite. If the stone does not have high thermal conductivity, it is likely not a moissanite or diamond.
  2. Check for a certificate inscription. The majority of diamonds have a microscopic engraving on their girdle which shows their certificate number. In addition, lab grown diamonds say “lab grown” or “LG”. Moissanite will not show this – so this is a great way to tell them apart!
  3. Check for doubling. When you look through the crown facets of a diamond, the culet (the bottom point) will be crisp and single. When you look at a Moissanite like this, it will look like it is doubled.
How to test for Moissanite using doubling instructions

CZ and Diamond Testers

Can CZ pass a diamond tester?

No. Cubic Zirconia (or CZ) does not have high enough thermal conductivity to pass. So, when you test a CZ with a diamond tester, it will not show as diamond. In fact, distinguishing CZ from diamond is one of the most important uses of thermal conductivity pen testers.

How can you tell a real diamond from a CZ?

Real diamonds test as diamond on thermal conductivity testers and most have microscopic laser inscriptions. Those are the two easiest ways to test between diamond and CZ. More nuanced tests include “pavilion flash” and “heft” – both of these require the stone to be loose. To test for pavilion flash, put your stone point side up and flash a penlight over it. If the stone flashes blue, the stone is likely CZ. And, to test for heft, weigh the stone in your hand. If it feels “too heavy” for its size, it is likely CZ (CZ weighs about twice as much as diamond).

White Sapphire and Diamond Testers

Do white sapphires pass a diamond tester?

No. White Sapphire does not have high enough thermal conductivity to pass. And, white sapphire looks quite different than diamond, moissanite, and CZ, so using a pen tester is not common. Whereas the high conductivity gems show rainbow sparkle, white sapphire shows white and silver sparkle. It also tends to have a silvery undertone. So, these are often easier ways to spot sapphire from diamond.

How could you tell a white diamond from a white sapphire?

Whereas diamond will pass a pen test and have rainbow sparkle, white sapphire will not pass and has white and silver sparkle. Sapphire is usually faceted differently than diamond, as well, with rectangular facets on the bottom of the stone. Finally, more nuanced tests include finding the “refractive index” of the gem. Notably, white sapphire’s refractive index also helps to distinguish white sapphire from white topaz!

Moissanite (top) vs White Sapphire (bottom) ovals

Can you answer more of my questions about diamond testers?

Sure! While we do not offer appraisal services or gem testing services, if you have questions about why your stone is testing the way it is, I’d be happy to answer them! And, as always, the best way to protect yourself in this once in a lifetime purchase is to choose a reputable jeweler to buy your diamond or moissanite from (hint hint: check out our reviews on Facebook, The Knot and Wedding Wire)!