|Q: How does one test a stone to see if it's cubic zirconia (CZ) or a diamond?
|A: The common method most gemologically untrained people use is a thermal probe type tester. These are available in a number ot types, but all measure the thermal conductivity of the material to which they are pressed. Diamond conducts heat very, very well, while most other gem materials are much more insulating in nature. These testers are quick and reliable for testing for C.Z.s, and most of the better ones will also tell you if you've mistakenly touched the metal of the mounting (which also conducts heat very well) and not just the stone.
With some practice (it's not hard) you can also learn to tell the difference just by looking at the stones, especially with a good 10x magnifier. Cubic zirconia, while a convincing substitute, is optically different, and you can see the difference in appearance.
They have much too much dispersion (fire), and this can be seen right off. While it's quite visible from the crown when a stone has too much fire, if you turn the stone over, and look at a light source through the pavilion, most (assuming reasonably good cutting) brilliant-cut C.Z.s will show a brownish-orange "flash" of dispersive color over much of the pavilion simultaneously. Diamond doesn't do this. Meanwhile, zirconia isn't quite as brilliant, and held next to a diamond, generally appears darker and less bright.
Under a magnifier, the fact that zirconia is much softer will instantly show up in the nature of the polish. Facet edges are just a little rounded over, while diamonds have sharp, crisp edges, even when facets are not so well polished. Looking at pavilion facet edges through the crown, those edges will often appear as slight lines, separate from the actual adjacent facets. The girdles of the stones will appear different as well, sometimes being simply ground. If polished, as they often are, they'll appear rounded a bit. If just ground, then they'll show grinding scratches. Diamond will not show grinding marks or rounding over. Overall, diamonds are a whole lot crisper and cleaner in appearance, especially under magnification. Only rarely will you see a zirconia that's so well cut and polished that under a magnifier it needs more than a moment's examination before revealing itself via the cutting differences. To the naked eye they can look very good, but under 10X....
And finally, most diamonds have inclusions. Zirconias don't duplicate that appearance, (though they can have fractures and chips) nor do they show the type of natural crystal surface textures on the occasional "naturals" that can be found on many diamonds, at the girdles. In a very few intentional attempts at fraud, such things have been mimicked, but that is very rare.
Hope this helps.