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 made to distinguish moissanite from diamond. A majority of diamonds have lower electrical conductivity compared to moissanite, however do note that there are both natural and lab-grown diamonds that can still fall within the same range as moissanite on some certain testers or units due to specific trace elements / impurities or from other reasons. Calibration and sensitivity on many of these testers can also often change over time (common). Despite this technology's marketing, we ourselves have seen many moissanite samples 'beep' or pass as diamond and vice versa on several electrical conductivity testers.
At our laboratory moissanite will never accidentally pass as diamond, as its anisotropic structure easily gives it away under microscopy observation by our graduate gemologists who have over a decade of experience participating in the jewelry trade. We also make use of photoluminescence spectrometry on every 'potential diamond' candidate to check if a stone's spectral absorption patterns. Moissanite will never pass this highly advanced method of testing.
We have also seen how many thermal and multi-tester readings can fluctuate in response to minor changes in the surrounding conditions, accidental contact with foreign surfaces, 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. Likewise miscalibration is definitely something to be wary of when using such kinds of testers.
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 "diamond tester" to be a supportive and helpful secondary test in gemology (when used in proper calibration), 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 never given us the confidence in consistency of results 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.
Such kinds of testers are alright as supportive tests, when performed honestly and correctly, but aside from not being fool-proof for certain case situations, fraudulent sellers and individuals can make use of 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. Many have manual calibrators that can also be manipulated in the wrong way.
Some vendors have even discovered faults in certain diamond tester units 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 consistent 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 calibrated condition) is better than simple guessing outright. However, we do not believe that these machines are 100% consistent in any way, all of the time. We have personally observed many misreadings with a varying range of causes when utilizing such machines, most of the time from unintentional circumstances.
We will not name any brands or models, as this post is not meant to demean the value of these instruments. They are still useful supportive tools in certain processes. These are strictly just the observations we've accumulated over the years in our industry. 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. We cannot stress this enough:
Multiple tests must always be considered without exception, so that you can avoid very costly mistakes.
There is no 'one-size' fits all 'DIY' test, each type of test has its own important limitations that if overlooked can lead to very costly mistakes by buyers and sellers, and so it's always professional SOP to counter-check each stone with multiple applicable tests for different identifying properties whenever possible. If you are in doubt regarding the findings of your tester, do not be alarmed, simply bring your stone to a gemological laboratory (one that employs a graduate gemologist and a lab-grade spectrometer, and preferably does not engage in the buying and selling of gemstones for non-bias / objectivity).
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.
