This article is only based on the opinions of our gemologist, as he has been steadily observing the habits and results of Filipino visitors who make heavy use of these instruments, and then bring their items to the laboratory due to their own uncertainty. You are free to conclude your own opinions regarding these testers, but the record below constitutes our experience of these portable machines.
It should be said that these types of pen-hold testers are also often called thermal testers as they are created to measure the thermal conductivity of a gemstone. In our own experience, we have had some consistent performance when using certain brands of testers that were manufactured by well-established companies, to separate regular cubic zirconia from diamond- however even these products (which may cost upwards of 14,000 Php) still made mistakes on certain stones like diamond-coated CZ or some other species of non-diamond gemstones.
In the past, we have also conducted experiments using diamond thermal testers that cost about 2,000-4,000 Php that were being sold here in the country, and also being imported from China-based manufacturers- a number of these testers occasionally failed on items like quartz, glass and cubic zirconia, passing them off as diamond. In terms of consistency, they were not dependable for our laboratory purposes. We purchased them strictly for the experiment only, and never put them into daily operational use here at the laboratory. (No specific brands or names will be mentioned to preserve objectivity and neutrality)
No standard diamond tester based on thermal conductivity (or even electric conductivity / multi-test pen-hold tools) has been known to be capable of separating synthetic or man-made diamond from natural diamond. These grown diamonds are also made up of carbon, with the same structure as natural diamond- but are grown by people in a factory. Advanced photoluminescence spectrometric testing like the technology we use here at our laboratory, is routinely utilized by industry professionals abroad for positively identifying the vast majority of natural diamonds by their trace elements and absorption spectra. Some natural diamonds may show inclusion-evidence of their unearthed-origin, and many older versions of synthetic diamonds may also show visual evidence of their man-made origin, however technology is getting better at a rapid rate, which causes alarm for many in the jewelry trade.
We hope that the general public here in the Philippines will use whatever personal tester they have with care, and a cautious, skeptical mind. The lab has seen too many obvious imitations brought in, mentioned as diamond by their owners, all because something beeped or a light flashed on their personal gadget.
In the end, if you would like to use these instruments on your own, spend the extra money to buy the more (or most) expensive ones. Those aren't perfect by any means- and have also been known to provide some mixed / wrong results if used by inexperienced checkers, but if used properly in the right conditions, they can help you at least with separating some kinds of lower-tier / cheaper imitations from diamond. This is still much better than buying the cheaper DIY testers that seem to cause a lot of unnecessary confusion.
