A thermal diamond tester measures how fast a stone draws heat from a hot tip. Diamond conducts heat very well, so it reads “diamond”.
In more detail
Moissanite conducts heat almost as well. In GIA's 1997 study, every moissanite read “diamond” on every thermal tester tried. So a thermal “diamond” reading means diamond or moissanite.
Related terms
- Electrical conductivity tester: An electrical conductivity tester checks whether a stone conducts electricity. Many moissanites conduct, and most near-colorless diamonds do not.
- Screening device: A screening device separates natural diamonds from laboratory-grown diamonds and simulants. Most answer Pass (natural) or Refer (send to a laboratory).
- Polariscope: A polariscope is a gem-testing instrument with a stone between two polarizing filters. With the filters crossed, it shows whether a stone is singly refractive, doubly refractive, strained or an aggregate.