Can a jeweller quickly spot a fake diamond?

A 10x loupe and a combined thermal-and-electrical tester catch most fakes in under a minute. Cubic zirconia and glass have softer facet edges, lower thermal conductivity and visibly different internal reflections; they fail both tests. Moissanite is the trickier simulant because its thermal conductivity sits close to diamond, so a thermal-only tester reads it as diamond. The electrical test is what separates them: moissanite conducts electricity, diamond does not. A jeweller looking through a loupe will also see the faint doubling of facet edges that gives moissanite away from the side.

Lab-grown diamonds are real diamonds, so they pass every test a fake would fail. Some HPHT-grown stones can occasionally read as moissanite on a basic electrical tester because trace metals from the growth process leave the stone slightly conductive; this is a tester false positive on a real diamond, not a sign of a fake. Confirming lab versus mined needs gemological lab equipment: short-wave UV fluorescence on instruments like DiamondView, or photoluminescence spectroscopy. A standard pen-style tester separates diamond from simulant in a few seconds, but it does not separate lab-grown from mined.