The Myths and Limitations of Diamond Testers

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Over the past few months our gemologists have had several people come up to us with a great diversity of materials, from moissanite to diamond-coated cubic zirconia, to synthetic lab-grown diamond. Many of them would say that a certain electronic diamond tester registered their item "within the diamond level", and therefore that indication supposedly led them to self-prove their item's natural authenticity.

The very first thing we would ask them (politely and respectfully) is, "If you're indeed certain about your item, and the credibility of your "automatic" tester, then why would you opt to come to our laboratory in search of further answers?". By this time, many of them would often reply something along the lines of 'taking a second opinion', or 'just having to be 100% sure of things'.



Here is a collective commentary from our gemologists who have been working in the jewelry trade for many years, even before Gemcamp's establishment as a gemological laboratory. Having been a part of jewelry companies in the Philippines, and in different parts of America, exposure to different brands and types of diamond testers was quite inevitable, and so logically tests, experiments and conclusions were drawn based on our concrete observations. These are not meant to be product reviews, so we will not be mentioning any specific brand or item, just our informed opinion of an widely marketed instrument that we have tested ourselves in many situations.

Let me begin by saying that we do have a diamond pen-type tester at the laboratory, patented and manufactured by one of the best instrument makers in the United States. It's a great tool that can help us separate some certain imitations from diamond, but even this tester is only ever used with extreme caution. Pen-type testers like these are divided into three types: Thermal conductivity testers, electric conductivity testers, and multi-type testers (both heat and electrical conductivity).

Thermal conductivity testers were the earliest ones invented, supposedly allowing people to separate their diamonds from the most prominent imitation at the time, cubic zirconia. When used properly in ideal situations, the testers would register a visual and audio signal denoting a pass or fail result for diamond authenticity.

Now, to put it plain and simple- moissanite, newer types of proprietary synthetic crystals, certain rough rocks and mineral compounds, and ALL synthetic (lab-grown) diamonds are capable of fooling thermal testers very easily, even if these are warranted to be in proper working condition. Thermal-conductivity based diamond testers can indeed designate some other materials as "diamond", even if these materials are composed of something else. This is because there are other gem-like materials that exhibit similar thermal conductivity properties comparable to diamond.

Next up, we have electric conductivity testers and multi-testers. These testers are slightly more advanced, normally being able to distinguish moissanite from diamond. Though despite this technology and moissanite's known difference from diamond in its measurable electrical conductivity, we ourselves have already witnessed many of these testers pass regular moissanite samples as diamond. We have also seen how tester readings can greatly vary depending on room temperature, recent human touch / contact with the stone, and even how hard you press the probe agains the material being tested. There are way too many influencing factors that could disrupt a test's performance.

Man-made synthetic diamonds, which are also made from pure carbon, will automatically register as "diamond" when tested with any electro-thermal conductivity tester. It is important to note that this test will not differentiate natural diamonds from synthetic ones.



While we do acknowledge the thermo-electric conductivity tester or modern "diamond tester" to be a supportive and important test in gemology, no gemologist from our institution would rely on the results of this test alone, even if he or she used a calibrated and quality-assured instrument from our laboratory suite. It has just never given us the confidence to do so. At best, instruments like these are simply used as a screener-test or secondary test for eliminating a few known diamond imitations.

Pen-hold testers are alright as supportive tests, when performed honestly and correctly, but aside from not being fool-proof in the best of conditions for every single stone, fraudulent sellers and individuals can make use of dirty tricks to purposely change a tester's result. The sensitive internal wiring of many pen-type diamond testers can sometimes change over time. They can also be dismantled and altered by external parties to reflect a higher sensitivity than recommended.

Some vendors have even discovered faults in certain diamond tester models that have allowed them to exploit the pressure differences of those machines to their advantage. Applying more pressure or less pressure can sometimes affect the resulting verdict ever so slightly, giving question to their accuracy relative to the motive of whoever is performing the test.

From our combined experiences in the jewelry trade, working with many of these machines, our conclusion and warning to the public is simple:

If you have no other means of testing a stone, using an electronic diamond tester (properly and in the right condition) is better than simple guessing. However, we do not believe that these machines are 100% accurate in any way, all of the time. They can indeed make mistakes, and we have seen a lot of them misread imitations as diamonds, so be careful.


We will not name any brands or models, as this post is not to demean the value of these instruments. It is strictly just the observations we've accumulated over the years in our industry. These machines are still useful to an extent, even in our own laboratory. For melee sized stones already mounted onto jewelry, a pen-type tester may be your only applicable test in specific scenarios. Like every type of test, whether it be for refractive index, pleochroism, optic character or thermal conductivity, there are always stones that can break the mould. Multiple tests must always be considered without exception, so that you can avoid very costly mistakes.



Be sure about your stones. There is a reason that gemological laboratories caution the usage of these instruments. Always take care that you apply the right amount of pressure in an environment that is conducive to proper testing.

Thermal conductivity and electric conductivity testers are objectively unable to separate natural diamond from synthetic diamond due to their same atomic structure and composition. More advanced spectrometric instrumentation is required to determine whether a diamond is natural or man-made.

Gemcamp Laboratories

A Discovery Institute for Gemstones and Gemology

Our resident gemologists believe in encouraging public trust within the jewellery industry through shared education, value transparency and professionalism.

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