Diamond look-alikes like cubic
zirconium,
moissanite and other synthetic stones, are distinguishable from nature diamonds based on their thermal and / or electrical conductivities. Germ testers that are on the market are capable of evaluating these two parameters as is the present invention. Electrical resistance of moissanites reaches hundreds of thousands megohms. Existing gem esters use test
voltage of 1000 volts, to be able to detect electrical
conductivity in most moissanites. Still, reliable detection of
high resistance moissanites is difficult. Proposed invention uses significant photo
conductivity of moissanites, which was observed by the inventors, to facilitate measurement of electrical
conductivity in the toughest gems, to reduce test
voltage applied to gems to 300 volts, and to limit electrical test current through a gem to no more than a few micro-amps. Other refinements include: (1) multistep evaluation of electrical conductivity, which avoids applying excessive or unnecessary test
voltage and current to a gem, (2)
circuit design, which efficiently attenuates AC
noise, (3)
signal processing, which eliminates industrial pick-up, (4) usage of reference temperature sensor, which improves sensibility and
repeatability of thermal measurements.