Zircon vs Cubic Zirconia

Zircon and cubic zirconia are both beautiful gemstones that are available in a range of colors and often used in place of diamond. Despite the similar-sounding names, the two gems are very different from one another. Here is a look at the differences between zircon and cubic zirconia, how you can tell the two stones apart, and how they compare with diamond.

Zircon vs Cubic Zirconia

Zircon and cubic zirconia both contain the element zirconium, but they have different chemical formulas and different crystal structures. The gems differ in their origins. Zircon sold commercially is a natural mineral, while cubic zirconia is synthesized in a lab. However, man-made zircon and natural cubic zirconia do exist. Both gems are available in a variety of colors, as well as colorless forms. One significant difference between their appearance is the zircon has less dispersion (fire or color play) than diamond, while cubic zirconia has more. Because zircon is a natural stone, it contains other elements besides just zirconium and oxygen. Traces of uranium and thorium make zircon slightly radioactive. Both zircon and cubic zirconia are thermal insulators.

What Is Zircon?

Zircon is a natural mineral. It is zirconium oxide, usually with the formula ZrO₄, but sometimes with a few atoms of other elements substituted within the crystal. Hafnium accounts for 1% to 4% of the mineral. Radioactive uranium and thorium are also present. Natural colors include colorless, brown, red, pink, yellow, green, blue, purple, and gray. Heating brown zircons turns them blue or colorless.

What Is Cubic Zirconia?

While cubic zirconia (CZ) does, rarely, occur within zircon crystals, the specimens are not large enough for practical use. So, cubic zirconia is grown in a lab. It has the chemical formula ZrO₂ and a different crystal structure and density from zircon. Because of its high purity, it is not radioactive. In pure form, it is colorless. Adding trace amounts of other elements produces a rainbow of color options.

How to Tell Zircon and Cubic Zirconia Apart

Zircon and cubic zirconia share several common properties. Both are beautiful, durable gemstones in their own right and inexpensive substitutes for diamonds. You can tell the two stones apart fairly easily.

• Radioactivity is a dead giveaway that the stone is a zircon.
• Both stones have a high index of refraction, so they sparkle as cut gems. But, zircon’s sparkle is white (similar to diamond), while cubic zirconia has a rainbow sparkle. Be careful, though, as moissanite and strontium titanate also have a rainbow sparkle!
• Set the stone on top of a paper with text. Zircon is birefringent, which doubles the image. Look closely, because careful cutting of a zircon minimizes this effect. Cubic zirconia does not display birefringence.
• Under magnification, CZ is pristine, with no inclusions. As a natural stone, expect some inclusions in zircon.
• Calculate the specific density of the stone. CZ is very heavy for its size, even compared with zircon.

Comparing Zircon and Cubic Zirconia to Diamond

Zircon and cubic zirconia are common diamond simulants. If you’re looking at them as cut gemstones, how do you tell them apart? The quickest and easiest method is also non-destructive. If the stone feels warm when you wear it, it’s diamond. If it feels cool, it’s not. Other good methods are density and optical properties.

• Mohs Hardness: Diamond is harder than either zircon or cubic zirconia and scratches either stones. Diamond has a Mohs hardness of 10, while CZ is around 8 and zircon is about 7.5. However, it’s unlikely you want to scratch your jewel, so you a different test.
• Appearance Under UV Light: Some diamonds (30%) fluoresce or glow under ultraviolet light, usually blue, but rarely green, yellow, or orange. On the other hand, cubic zirconia and zircons generally glow mustard yellow or greenish under a black light. Note that heat-treated zircons (to make brown ones clear or blue) sometimes revert back to their original color if you expose them to UV for too long!
• Thermal Conductivity: Both zircon and cubic zirconia have remarkably low thermal conductivity values. In other words, they are thermal insulators. Diamond, on the other hand, is one of the best thermal conductors. You can’t use thermal conductivity to tell zircon and cubic zirconia apart, but both stones feel cold to the touch. When you hold or wear diamonds, they feel warm because they conduct body heat.
• Density: Both zircon and cubic zirconia are very dense. They readily sink in liquids, even viscous ones like honey. Diamond is less dense. Measuring the density of a stone is a good way to tell read diamonds from simulants.
• Optical Properties: All three gemstones have a high index of refraction. However, zircon is birefringent, which gives it double refraction. Cubic zirconia has more fire or dispersion than diamond, which means its sparkle is more colorful than a diamond’s.
• Radioactivity: If you happen to have a Geiger counter sitting around, it’s easy identifying zircon because it is faintly radioactive. Diamonds and cubic zirconia generally are not.

References

• Angus, J. C. (1997). “Structure and thermochemistry of diamond”. In Paoletti, A.; Tucciarone, A. (eds.). The Physics of Diamond. IOS Press. ISBN 978-1-61499-220-2.
• Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (1995). “Zircon“. Handbook of Mineralogy. Vol. II (Silica, Silicates). Chantilly, VA, US: Mineralogical Society of America. ISBN 978-0962209710.
• Dhanaraj, Govindhan; Byrappa, Kullaiah; Prasad, Vishwanath (2010). Springer Handbook of Crystal Growth. Springer. ISBN 978-3-540-74761-1.
• Wise, R.W. (2016). Secrets of the Gem Trade: The Connoisseur’s Guide to Precious Gemstones (2nd ed.). Brunswick House Press. ISBN 978-0-9728223-2-9.