How diamond’s properties can be used to differentiate it from fakes

Diamonds are a girl’s best friend.
Marilyn Monroe

Yep, someone probably said it before her. And however questionable you find it, diamonds find themselves among the most expensive gemstones in the world, and it’s obvious why. With a low critical angle, light is totally internally reflected inside a diamond many times which is what gives diamonds their super sparky appearance. But how are jewellers able to identify between the real deal and the fakes?

Among the most common fakes are cubic zirconia and moissanite, which are both cheaper, more durable, and similar in appearance alternatives to diamonds.

There are several tests to tell a real from a fake, some professional jewellers may use, some not. Below I’ll outline the tests and explain, referencing the structure of the stones why these tests (might) work.

Water test

Drop a diamond into a glass of water. Diamonds have a high density so will sink, while fakes (such as glass or quartz) might float at or just underneath the water surface.

As first introduced in GCSE, each carbon atom is diamond is bonded to 4 others, giving a tetrahedral arrangement of atoms with a 109.5 degree bond angle and a 1.5 x 10^-10 metres separation. Because atoms can pack in so closely, diamond has a high density, higher than other allotropes of carbon such as graphite.

That said, cubic zirconia, depending on its crystalline form (monoclinic, tetragonal or cubic), can have a density ranging from 5.83 to 6.09 g cm^-3, while diamond generally has a density of only 3.51 g cm^-3. Therefore, this test won’t work differentiating cubic zirconia from diamond.

However, if you know the volume and weigh the stone, you can use this to work out the density which can be used to differentiate instead (although arguably taking more time!).

Fog test

Breathe on the diamond for a few seconds until a fog forms on the stone surface. With a real diamond, the fog will disappear almost immediately, with a fake the fog hangs around for a few seconds. This is due to diamonds being much less able to retain heat. They are incredible heat conductors- much better (3 to 4 times better) than metals like copper or silver. When breathed on, diamonds heat up very quickly and the fog dissipates very quickly (as the diamond is not longer cold the water does not condense on the surface).

Similar is using a thermal conductivity probe (also known as a “diamond tester”). As mentioned above, diamonds are effective thermal heat conductors so they disperse heat quickly after being warmed. This is a tool to conduct the fog test with greater accuracy. A thing to note: moissanites have very similar heat dispersement properties, so both the fog test and a thermal conductivity probe won’t be able to differentiate between the two.

Heat test

Heat the stone using a flame then drop the heated diamond into cold water. Diamond will not shatter. A fake diamond (made of weaker components such as cubic zirconia or glass) will. This is due to the sudden expansion (from the heat) and contraction (from the cold). With diamonds, heat disperses quick enough to prevent this occuring.

UV light

When placed under a UV light, (most) diamonds will emit a blue light due to the natural fluorescence of diamonds. Fluorescence is defined as the emission of light by a substance that has absorbed light or other electromagnetic radiation. Here, the electromagnetic radiation is the UV light, and the emitted light is the blue light.

The minerals in diamonds which cause this blue light effect are aluminum, boron, and nitrogen. Not all diamonds emit blue light under UV light- these wouldn’t contain any of the elements listen above. So it’s not a sure fire test. The UV light photons are absorbed by the electrons orbiting the diamond’s trace aluminium/boron/nitrogen atoms. The electrons are excited to a higher-energy level. But what comes up must come down: when the electrons de-exite (return to their original energy level) they emit light of energy equal to the difference between the energy levels. E = (h x c)/wavelength can be used to find the wavelength (a.k.a colour of the light emitted) from the energy.

I think this video is pretty cool, if a bit out of focus, while this one has a little bit more of an explanation behind it.

Newspaper Test

As mentioned above, light passing through diamonds totally internally refracts a lot. This is the principle behind the newspaper test. Place a stone on a newspaper. If it’s a fake (e.g. cubic zirconia), you should be able to see right through and read the writing (though it may be a little blurry). Using a diamond means you won’t be able to read the words.

Electric Conductivity Test

This test is good for differentiating between diamonds and Moissanite and they have similar levels of thermal conductivity so other tests may not work as well. Diamonds won’t conduct electricity, moissanite will. This is because all of carbon’s four outer electrons are used in bonding (unlike in graphite which does conduct electricity), therefore there are no delocalised electrons to carry a charge.

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