The only material other than diamond that shows as a diamond on
the thermal diamond tester is moissanite (sp?).
Moissanite can be differentiated from diamond by looking,
through the table of the stone, at the facet junctions near the
culet with a loupe. Moissanite is doubly refractive, diamond is
singly refractive. A moissanite stone will show doubling of the
facet junctions, diamond won’t. You may have to look at several
of the facet junctions to see doubling if it is present.