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Diamonds: How to tell CZ from Diamond

Diamond simulants have been around forever. Synthetic Cubic Zirconia, or CZ, is one of the newer ones, and today is the most popular of them all. They look like diamonds to the casual observer, but they have very different properties and can easily be distinguished from real diamonds by simple tests. Certified diamonds are screened for simulants and synthetics with sophisticated instruments as one of the first steps in the lab grading process.


When CZ first appeared on the market in the late 1970’s, it was relatively expensive among gem simulants and synthetics. But like all man-made gemstones, prices rapidly fell to the point where CZ is sold at retail for very little money today, and has found favor mainly in costume jewelry.



Simple Tests to Tell CZ from Diamond

The following tests can help the layman tell a CZ from a real diamond.


  1. The weight test: If the stone is loose and can be weighed it is very easy to determine if it is a CZ as it will weight approximately 1.7 times as much as a diamond of the same dimensions. For instance, a round diamond that is 6.5 mm will weigh approximately 1.00 carat. A CZ this size will weigh about 1.70 carats.

  3. The fog test: If you breathe on the stone, as you would to fog a mirror, a CZ will stay foggy for several seconds while the fog on a diamond will dissipate almost immediately. This is due to diamond having much better thermal conductivity.

  5. The newsprint test: This test also requires that the stone be loose. Lay the stone upside down on a page of the newspaper or a magazine. Due to the high refractivity of diamond (it bends light more so than CZ) you will not be able to distinctly see the letters, whereas with a CZ you will be able to see parts of the print.


Other Clues to Tell if a Diamond is Real

Some other clues that a stone might be a CZ or other simulant are the following:


  1. If the setting is base metal, gold plated, or silver it is very unlikely to be set with real diamonds. Look for stamps such as 14K, 18K or PLAT that would indicate the setting to be precious metal. That is by no means conclusive –CZ can be set in any metal – but it is an indication.

  3. If the stone is colorless and has no inclusions visible under magnification, chances are it is not a real diamond. This is not conclusive because some real diamonds are colorless and flawless.

  5. Under magnification if the facet junctions are slightly rounded or the stone has any obvious abrasions, it is most likely NOT a real diamond. The extreme hardness of diamond enables it to be faceted to very precise edges and it is unlikely to be heavily scratched or scuffed in wear.


Other home tests are possible, but when in doubt, taking the stone to a reputable jeweler is always best. Most jewelers have handy electronic testers that can quickly identify a CZ as a simulant. A trained gemologist can perform the tests necessary to make most important distinctions. And they can assist you if the stone warrants sending to the laboratory for more advanced testing or grading.


Synthetic Diamonds

Synthetic diamonds are another type of man-made stone, and like the others has very little resale value. However, they can be more difficult to identify as they have the same physical properties of natural diamond. Synthetic diamonds may be laser inscribed with their man-made status. Inscriptions can be read under magnification and is, in most cases, something a jeweler would be able to easily observe.


What to Look for in a Real Diamond

Natural diamonds are the king of gems for a reason. They are rare, beautiful and valuable. They have lasting value because the supply of natural diamonds is limited and because mining and cutting them is expensive. Values are driven by the diamond 4C’s; Cut, Color, Clarity, and Carat. For a primer on what to look for in a real diamond please see this diamond buying guide.


Diamond Beauty - the Quality of the Cut

While diamonds are rare and durable, it is their brilliance, fire and sparkle that makes them so beautiful. And this beauty is dependent on diamond craftsmanship and cut quality which determines the light performance of the diamond.


Ideal cut diamonds are those that possess the baseline qualities necessary to gain a cut grade of Ideal from the AGS Laboratories. Diamonds that surpass these requirements are referred to as “super ideal”. A CUT ABOVE® Super Ideal Diamonds in both round hearts and arrows and princess cut, are considered by many experts to be among the finest diamonds in the world. Extreme faceting precision results in diamonds with extraordinary light performance and beauty.



For more specific questions ask our experts
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