Testing Lab-Grown Diamonds
History of Diamond Testers
A handheld diamond tester typically works by measuring the thermal conductivity of the stone being tested. As you mentioned, diamonds have a very high thermal conductivity, while diamond alternatives like cubic zirconia and moissanite have much lower thermal conductivity. When a diamond tester is used on a diamond, the heat from the probe is quickly dispersed throughout the stone, causing the tester to beep or light up to indicate that the stone is a diamond. In contrast, when the probe is used on a diamond alternative, the heat is not dispersed as quickly, and the tester will not register the stone as a diamond.
New Testers Developed In Response to Moissanite
When Moissanite came on the scene in the 2000's, it rendered the thermal diamond probes useless because it is an excellent conductor of heat. Therefore it would falsely test positive as diamond. In response, new testers were developed that also tested how well the gem conducted electricity. Whereas Moissanite conducts heat and electricity well, most diamonds are not good electrical conductors. So based on how well the gem conducts heat and electricity, the tester makes a guess as to the identity of the gem.
Flaw In The Way Diamond/Moissanite Testers Work
It's important to note that relying solely on a handheld tester to identify a gemstone can be problematic, and a trained gemologist should always examine the gems to ensure an accurate identification. It's also interesting to learn about the different electrical conductivity of lab diamonds and Moissanite and the varying results obtained from handheld testers.
The problem with differentiating between Moissanite and diamonds based solely on whether or not it conducts electricity is that there exists a type of diamond that is electrically conductive. It's technically known as a "type 2" diamond. Due to the existence of trace amounts of boron in the diamond crystal, type b diamonds conduct heat and electricity, and so they can falsely be identified as Moissanites by handheld testers. Does that mean they are really Moissanite? No! It means the diamond tester is wrong.
The lab diamonds sold by MoissaniteCo.com are type 2 and made via HPHT technology which makes colorless diamonds possible. In our opinion, they are the best quality lab grown melee diamonds on the market. As type 2 diamonds, they are electrically conductive and can falsely be identified as Moissanite. However, a trained gemologist should be able to differentiate between lab diamonds and Moissanite via examination of the gems. For example, Moissanites are doubly refractive whereas diamonds, natural and lab, are not, and a jeweler should easily be able to observe this under magnification. (Sadly too many jewelers use the diamond/Moissanite tester as a crutch and rely on it exclusively).
In our own testing, we have found that not all lab diamonds were falsely identified as Moissanite on handheld diamond testers: some falsely identified as Moissanite, some as diamond and some were even identified as "metal" (which is an option on the tester). We believe this is due to varying electrical conductivity from stone to stone (with metals being the most electrically conductive of them all). Our hypothesis is backed up by the GIA article linked to at the end of this article. Unfortunately given how new these stones are to the market, little academic research has been done in this regard.
Below is a video showing a 0.25ct round brilliant lab diamond being "tested" with a Moissanite / Diamond tester (GemOro UltraTester II).
You will notice that when the lab diamond is in the plastic tray, it tests as a diamond. However, when touching the metal tray, it falsely is identified as a Moissanite. The reason? Given the trace elements of boron in the lab diamond, it is already slightly electrically conductive. Once it is touching metal, it helps complete some electrical loop, enough that the tester fails to identify it correctly.
Conclusion: Moissanite testers aren't currently reliable and will not accurately identify lab diamonds.
For additional reading: