How to Tell a Diamond From a Cubic Zirconia

Using Physics to Identify Different Gemstones

The cubic zirconia is a reasonably good "fake" diamond – although calling it a fake diamond is perhaps a little unfair – the cubic zirconia is a gemstone in it own right, referring to it as a fake diamond makes it sound like the glass "stones" used in costume jewelry. The reason that it is possible for a cubic zirconia to pass as diamond to casual or untrained scrutiny is that the cubic has many of the properties of a diamond – they are the same shape and color. However, there is one important property of diamonds that a cubic cannot mimic well – the "sparkle."

Diamonds have a very high refractive index – this means that light entering or leaving a diamond will be bent strongly as it crosses the diamond-air interface. The amount of bending is given by Snell’s Law:

Sin(theta_i)/Sin(theta_r)=n_i/n_r

The ratio of the sines of the angle of incidence and refraction is equal to the ratio of the refractive indices on the incident and refraction side of the interface.

Thus, we can see that when light travels from diamond (n_diamond = 2.42) to air(n_air = 1) the sine of the angle made by the beam of light in the air is greater than the angle made in the diamond.

Further, you can see that the, above some critical angle of incidence, the "sin(theta_air)" given by Snell’s Law will be greater than 1. As this is impossible, we know that above the critical angle, we cannot have refraction. But the light has to go somewhere – it is reflected back into the diamond.

In a cubic zirconia, the refractive index is smaller (n_cubic = 2.14) so the critical angle in the cubic is greater.

The upshot of this is that more of the light that enters a diamond from the front is reflected from the back than if the gem was a cubic zirconia (or pretty much any other gemstone) – giving the diamond its characteristic "sparkle."

However, it is not easy for someone who is inexperienced with gemstones to tell by eye whether a random clear stone has enough sparkle for it to be a diamond. However, there is an easier way to see the refractive index of the gem stone, allowing you to tell a diamond from a cubic.

We see the clear gemstones in the air because of the light that reflects off their faces. The amount of reflection is related to the difference in refractive index between the gemstone and air (n_air=1). In this situation, the cubic and the diamond reflect a very similar amount of light. But, if you submerge the gemstones in water, the ratio of the refractive index differences increases markedly.

Thus, if you submerge a cubic zirconia in a glass of water, the gemstone virtually disappears, while a diamond is still easily visible.