A professional gemologist on another forum related that a particular top-of-the-range one seemed reasonably accurate, don't know what that brand was nor how much it cost.
Thermal conductivity is real and measurable and different gemstones each have their own ranges. Sapphires have a high thermal conductivity which I noticed when trying to polish one dopped with wax on a ceramic lap. The friction generated heat which doesn't disperse that well through the ceramic lap but is conducted effectively by sapphire. Result: the stone heats up, transfers the heat to the dopping wax which then soften and the stone turns out of alignment on the dop right at the final stage
I love my ceramic polishing laps and use them for nearly everything but it's only sapphire that I've had that problem with so I dop them with epoxy only.
As long as this device is reasonably accurate at measuring what it's supposed to then I think it could have a place as
part of an identification tool kit. I wouldn't rely on it alone - I would want to test any result it gave against other indicators like relative density, refractive index, even a simple scratch test. It's probably really fast to give a result - probably a couple of seconds - and anything that gave a reading would be worth further examination. But yes, an experienced eye is one of the best things.
Prospectinglife, do you have a pointed quartz crystal? You should find it all but impossible to scratch a sapphire with quartz.