Diamond Simulants: Cubic Zirconia

Cubic Zirconia

To the average consumer, Cubic Zirconia (CZ) is the most familiar type of diamond simulant on the market. While a synthetic diamond is a man-made recreation of an actual carbon-based diamond, Cubic Zirconia (Zirconium Oxide ZrO2) has a completely different chemical structure, and CZ has a hardness of only 8.3 on the Mohs scale, while diamond has a hardness of 10. The toughness of Cubic Zirconia is rated as good.

In 1973, Soviet scientists at the Lebedev Physical Institute in Moscow perfected the technique of manufacturing cubic zirconia via the "Skull Crucible" process (photo below left). Zirconium oxide powder is heated, then gradually allowed to cool in the crucible. Once the mixture has cooled, the outer shell is broken off (photo below right) and the interior core of the "run" is used to make the final cut stones. The original name for cubic zirconia was "Jewel Fianit," but this name was never used outside of the Soviet Union (USSR).
Unlike most natural diamonds, a CZ is optically flawless. Cubic Zirconia can be made in both colored, or colorless (white) versions, and CZs can be made in any "color grade, although D-colorless versions are more expensive to produce. The 1.80 to 2.17 refractive index (RI) of Cubic Zirconia is lower than a diamond's 2.41 (RI). Due to their low cost and consistency, CZ color-grading sets are used to do a comparative color analysis of natural diamonds.

Photos: Larry P Kelley

Identifying Cubic Zirconia

A trained gemologist will easily be able to distinguish a natural diamond from a synthetic CZ diamond, but there are visual differences that can be detected with the untrained eye. For instance, the greater dispersive power, or "prismatic" effect of CZ creates an abnormally high amount of fire when compared to natural diamond.

CZ vs Diamond - Detectable to the Naked Eye
  • Light Dispersion: Greater prismatic effect of CZ creates an abnormally high amount of fire
  • Lack of Flaws: CZ is virtually flawless, most natural diamonds have some inclusions
  • Color: CZ can take on a gray tone when exposed to sunlight for prolonged periods
CZ vs Diamond - Detectable With Testing
  • Fluorescence : Under shortwave UV light, CZ typically luminesces a greenish yellow color
  • Refractive Index: Cubic Zirconia refractive index of 1.800 - 2.170, Diamond's RI is 2.417
  • Thermal Conductivity: CZ is a thermal insulator, natural diamond is a thermal conductor
  • Weight: Cubic Zirconia is heavier than diamond in a given size
Another method for distinguishing Cubic Zirconia from diamond is to mark the stone with a grease pencil or felt-tipped pen. Natural diamonds attract grease, while a Cubic Zirconia will repel grease. You can also Inspect the facet edges with a 10x loupe to look for any chipping or slightly rounded (not sharp) facet edges that are telltale signs of Cubic Zirconia.

Using Visual Optics To Detect CZ

The Hodgkinson "Visual Optics" technique was developed in the mid-1970s by Alan Hodgkinson as a method of detecting natural diamonds and diamond simulants. When holding the crown or table of a stone close to your eye while squinting, you would look towards a single pinpoint of light (pen flashlight, candle, etc.) in a totally dark room. You must be at least 10 feet from the light source when observing. The patterns will be very different for each type of stone (see samples below).

A small, sharply focused pattern seen in natural diamond is caused by secondary reflections due to a diamond's high refractive index. A significantly larger pattern will be visible in Moissanite due to its similarly high RI, while Cubic Zirconia's pattern will be diffused, being caused by primary reflections bouncing off of the inside surface of the pavilion. CZ's has a low RI compared to Moissanite or diamond.

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