Presidium Gem Testers

I’m curious if anyone has a Presidium PGT II gem tester and how much it can be relied upon for your gem identification pursuits? I’ve taken nearly the entire IGS certification course and learned about a great multitude of gemological testing equipment, but the Presidium gem tester is not mentioned at all (nor is any other brand if they exist), so I’m wondering if this tool is taken seriously by gemologists? It obviously doesn’t identify EVERY gemstone but it seems to be able to identify quite a few popular ones and can distinguish between diamond/moissanite and glass. Do gemologists use these in their work at all?


I am not a gemologist, but I do have my GIA AJP diploma. I too am still learning gemology. I do own and use a Presidium Instruments Gem Tester II and a Presidium Refractive Index Meter II that has a computer software and database to work with.
When I buy my gems from my sellers, I usually use the Gem Tester II first, then next weigh, measure, calculate SG, verify color with Gemworld’s GemGuide, UV shortwave light &/or filters if warranted. I do use a Diffraction Spectroscope. Last, I use the RI tester that is connected to my computer… This last piece of equipment helps a lot when there are many possibilities of what a gem could be.

I will say that the Gemworld’s GemGuide color check for a specific type gem (usually sapphire or spinel) helps me to see if a gem is a lab created stone or not. Usually the color gives it away. Still saving up for a high quality gem microscope. That will help with seeing the internal crystal structure.

If I think a sapphire or spinel is lab created, I send it into the gem lab to be tested & certified.

Even though I am not a GIA GG, I like my tools listed above, plus my standing 30X loupe. I hope I helped a little to answer your question.

I completed the gemologist course over a year ago (great experience - really learned a lot). I looked into the presidium, but decided not to purchase it. Instead I purchased a higher quality refractometer, a good portable scale; a set up to determine s.g., and a Gemax microscope. Learning how to use the refractometer correctly as well as the scale to assess specific gravity were keys to success.

With the right tools I was able to complete the practical examination with 100%. If you want to pursue a career in gemology, there really isn’t a short cut and the better the tools the better the results.

I checked with multiple people who have the presidium and they agreed that it helped identify or confirm the RI about 60% of the time. It also helped to rule-out other possible gems. But they still had to do additional assessments to confirm the presidium’s results.

I plan to take additional gemology courses, and having reliable tools will be important. I am not totally against the presidium, but I am not convinced it is reliable enough.

I wish you well.

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That’s a great question! The Presidium Gem Tester II can be a useful tool for initial gemstone screening, but it’s important to understand its limitations.

You’re right, the IGS certification course likely focuses on more sophisticated gemological equipment because the PGT II relies on thermal conductivity testing, which has some inherent drawbacks.

Here’s a breakdown of why the PGT II might not be considered a definitive tool by professional gemologists:

  • Thermal Conductivity Dependence: The PGT II measures a gemstone’s heat transfer rate. This can be helpful for separating diamond simulants like moissanite from diamonds (both have high thermal conductivity) or for differentiating gemstones with very different thermal properties. However, many gemstones have similar thermal conductivities, making precise identification difficult.
  • Calibration Challenges: The accuracy of the PGT II depends heavily on proper calibration, especially of the probe tip. As you mentioned, even slight temperature changes from touching the stone can affect readings. Professional gemological labs use more controlled environments to minimize these variations.
  • Limited Scope: The PGT II provides a basic identification based on heat transfer. It doesn’t tell you much about a gemstone’s internal characteristics, inclusions, or treatments, which are crucial for a full gemological assessment.

In essence, the PGT II is a good starting point for initial sorting or quick screening. But for professional identification and certification, gemologists rely on a combination of tools including refractometers, polariscopes, and Chelsea filters, along with their experience and training.

Here’s an analogy: Imagine the PGT II like a kitchen thermometer. It gives you a general idea of the temperature, but for precise baking, you’d use a professional oven with more controls.

So, while the PGT II might not be a definitive tool for professional gemologists, it can be a helpful addition to your gem identification kit for preliminary screening!


Presidium Gem Test is the best for identification of natural diamond from other simulant ( cubic zirconia maily and other gems.) Is also best tester for identification moissanite and HPHT and CVD sintetic diamonds by the different electrical contuttivity.
Dr. Piero Manuelli FGA CEO of ATC Italy of Gem-A The Gemmological Association of Great Britain

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gem testers measure thermal and electrical conductivity. As noted above, it’s best suited for separating diamonds from simulants such as moissanite and CZ. It can’t separate natural from synthetic diamonds, nor other synthetics with the same crystal structure and chemical constituents, and requires meticulous calibration. This discussion has come up before. It’s a useful screening test, but in no way definitive, in and of itself. Other techniques still have to be used to confirm the nature of a stone. This includes separating synthetics from natural, which is difficult and relies on identification of inclusions characteristic of synthetics, depending on the method by which they were grown.

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