It depends on the gem in question.
Diamonds for example are REALLY good conductors of heat. Absurdly good compared to most other things that look like diamonds. So cheap “diamond testers” just try to measure that property. However there because there is one diamond look-alike that will fail the heat conductivity test (Moissanite), so better (more expensive) diamond testers use electrical conductivity to account for that.
A series of non-destructive tests or destructive tests. electrical or heat conduction, hardness, luster, streak, color, cleavage, specific gravity, magnetism, taste/texture to tongue. Also experience, a Sr. jeweler can tell what a stone might be based on its setting.
There is a different test and scale for each measurement.
gemological institute:
[https://www.gia.edu/](https://www.gia.edu/)
[https://www.gia.edu/gem-education/course-gem-ident](https://www.gia.edu/gem-education/course-gem-ident)
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