Check for a clear, bright sparkle with strong white and colored flashes
Look for high transparency and minimal visible inclusions
Test hardness by seeing if it scratches glass or metal easily
Examine the edges for sharp, crisp facets
Check for a high refractive index and noticeable brilliance
Use a loupe to inspect for natural inclusions and facet junctions
Observe whether the stone feels cool to the touch at first
Verify that it does not fog easily when breathed on
Test thermal conductivity with a diamond tester
Compare weight with similar-sized stones, since diamond is relatively dense
Inspect for doubling of facet edges in some imitation stones
Check for fluorescence under UV light
Request a professional gemological certification
Have a jeweler perform a refractive index and specific gravity test