Bench model. A dual tester with both thermal meter and reflectivity meter.
Place the probe on the stone to operate analogue meter (the thermal meter, see detail in the extra pictures).
Place the stone over the sensor (even if it's in jewellery) to operate the digital meter (the reflectivity meter).
You don't have to use both systems, you can choose one or the other, or both!
The thermal meter works with any stone, loose or mounted in jewellery, even uncut stones. The only condition is that the stone (and tester) must be at room temperature. It is the same as the basic Presidium model (thermal meter only), it tests for 17 common groups of gemstones (about 40 varieties), enlarge the product picture to see the scale in detail. The reflectivity meter works on stones with a perfectly flat, perfectly polished surface, than can be rested over the sensor, it is not affected by temperature.
Have you seen the fascinating article about gem testers? This will answer many questions about electronic gem testers (all electronic gem testers!) - how 'reliable' are they? How 'easy to use' are they?
In summary: no gem tester will light up with a number (or name) that tells you, with 100% certainty, the name of the gemstone. This is because nature does not make gemstones to a standard formulae, there are usually a few gemstone with the same reading. All of gemmology is a detective game. Start by using an electronic gem tester to narrow the possibilities from several to two or three, then use your knowledge to find the answer.