IIRC they either measure resistance or heat conductivity (as a carbon crystal structure has very specific properties). The device was a pen with two probes on i

IIRC they either measure resistance or heat conductivity (as a carbon crystal structure has very specific properties). The device was a pen with two probes on it. It had different lights that said "Diamond", "Mossanite", and "Cubic Zirconia", so I was very surprised to see that the detector thought it was a diamond.