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Diamond Simulants: How to distinguish diamond from CZ and synthetic Moissanite?

With their beauty, durability and rarity, precious stones have a great power of attraction for humanity. Although the range of gemstones available is wide, four of them are the most important in the gem business: diamond, ruby, sapphire and emerald – and of those, diamond is foremost. Its great value and exceptional properties have meant that people have long searched for other materials that exhibit these same properties at a considerably cheaper price. 

Natural diamond simulants, which can be found in nature and are not man-made, are relatively rare. The most important are: colourless zircon, which is considered an “inferior” diamond. It has been mined for over 2000 years in Sri Lanka; colourless topaz and beryl are also natural diamond simulants, but mass extraction and production is unfeasible as they contain many inclusions. Consequently, the majority of diamond simulants are artificial.

The oldest known form of artificial simulant is glass, used as “fake diamond” since 1500. Colourless synthetic sapphire (marketed as Diamondette or Diamondite) and spinel appeared in the early 1900s and were followed, with mixed success, by a whole range of other imitations. To name a few: synthetic rutile, strontium titanate (fabulite), yttrium aluminium garnet (YAG) and gadolinium gallium garnet (GGG). Some of these are still made and used at the lower end of the market.
Today, the two most important diamond simulants are Cubic Zirconia and Synthetic Moissanite.
What are diamond simulants, and what is the difference with laboratory-grown diamonds? 
Simulants are minerals or other materials that only copy some characteristics of the diamondThey look like diamonds but are not diamonds. Their physical, chemical, and optical properties are different than those of diamonds, enabling them to be distinguished from diamonds. 
Lab grown diamonds are products created by a technological process in a laboratory, not by nature. They have essentially the same chemical, physical and optical properties as natural diamonds. The term “laboratory-grown” diamonds equals “laboratory-created” and “synthetic” diamonds.
In this article, we will introduce you to the two most important diamond simulants, Cubic Zirconia and Synthetic Moissanite.

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Cubic Zirconia (CZ)
From 1970 to 1990, Cubic Zirconia (CZ) was the best and the most important diamond simulant. In 1937, two German mineralogists discovered it, but it wasn’t until 1970 – in a Russian laboratory – that it was produced artificially. Cubic Zirconia is usually yellow, light green or brown. Heat treatment has made these gemstones available in a colourless form.
Cubic Zirconia (CZ) is handled in bulk, first appearing on the market in 1976 and immediately ousting all other diamond simulants (including YAG and GGG). CZ looks so much like diamond that an inexperienced person or even a jeweller can mistake it for the real thing.
The production of high quality Cubic Zirconia is an art in itself. Extremely high temperatures are necessary in order to melt the essential ore. The cooling process requires very strict control to obtain perfect crystals.

How to identify CZ?
CZ is hard to detect visually, but there are methods one can easily use to receive an indication that a stone might be a CZ (it could also be another type of simulant), and not a natural diamond:

  • Due to its lesser degree of hardness, and as a result of daily wear as a jewel, CZ rapidly shows external damage. This can be a sign that one is not dealing with a natural diamond.

  • If one looks at a line through the table plane of an unmounted round brilliant diamond, the line is very difficult or impossible to see, due to the refractive index of the diamond. If, on the other hand, one looks through a CZ, the line is clearly visible. Be aware that this test can only be used on unmounted brilliant-cut stones (not for fancy cuts). If you see the line clearly, you can be a 100% sure you are not dealing with a natural diamond. On the contrary, if the line is very difficult to see, you cannot be 100% sure you are looking at a natural diamond (it could be a Synthetic Moissanite).

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  • Thermal conductivity is a common method for distinguishing diamond from its imitations, which tend to be thermal insulators as opposed to conductors. Diamond is the best thermal conductor on earth, meaning that heat spreads very fast across the entire stone. Reading the thermal conductivity with a diamond tester is very simple: the probe applies a quantity of heat to the stone, and then measures the speed with which the heat dissipates or, in other words, how quickly the needle cools. CZ can then easily be distinguished from diamond, as its thermal conductivity is very low. However, like diamond, Synthetic Moissanite has a high thermal conductivity; consequently, many gem testers might also interpret this simulant as a diamond.

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As mentioned above, these methods will not prove that one is dealing with a CZ, but they will let you know you are not dealing with a natural diamond. 
There is in fact one unambiguous method of distinguishing a loose round brilliant diamond from a loose round brilliant CZ, and some other of its imitations: the determination of density method. The mass density of diamond is constant, whereas that of most simulants tends to be very different from that number. Cubic Zirconia has a much higher density (5.65 g/cm3 compared to 3.51 g/cm3), and thus a brilliant cut with the same diameter weighs 1.6 times more than diamond.
The weight of a diamond can be determined theoretically using the following formula:

Weight (ct) = Max diameter X Min diameter X Height x k

  • The Maximum and Minimum diameter as well as the Height are given in millimetres.

  • The value k” is a rounded off value containing information on the volume and density calculated for a brilliant diamond, as well as the conversion from grams to carats. The average value for k is 0.0061; but this can vary if the stone has a greater total depth.

Once you have used the formula to calculate the theoretical weight of the diamond, measure its weight by weighing it on a scale. If the stone is a natural diamond, both methods will give an identical weight. If the theoretical weight is not identical to the weight given by the scale, you are dealing with a diamond simulant, potentially a CZ.
This procedure is only possible, however, for simulants whose density differs significantly from diamonds, like CZ. For example, Synthetic Moissanite cannot be identified with 100% certainty: the density difference is relatively small (3.51 g/cm3 vs 3.2 g/cm3), which means that the value will fall within the calculation’s margin of error.

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Synthetic Moissanite
The most recent diamond imitation is Synthetic Moissanite. The US company, C3 Inc., developed and put it on the market in 1997. Since then, many manufacturers entered this market.
Natural Moissanite is a mineral that was discovered in fragments of the meteorite that hit Diablo Canyon (Arizona). It was named after its discoverer and Nobel Prize winner, Dr. Ferdinand Henri Moissan. Natural Moissanite is extremely rare and only appears in relation to iron-nickel meteorites and a couple of rare ultramafic-depth stones. To our knowledge, Natural Moissanite has never been found in significant sizes and good quality.
Synthetic Moissanite, also called silicon carbide, has fooled many experienced jewellers with its very high hardness (9.25 on the Mohs scale) and high refractive index (2.6 – 2.7). Synthetic Moissanite has a very high thermal conductivity, which has rendered some tests useless.

How to identify Synthetic Moissanite?

  • From certain viewing angles, you will find a double refraction in Synthetic Moissanite. This double refraction is visible through the crown with a loupe 10x and certainly with a microscope. This feature is always present in Synthetic Moissanite (as well as in other simulants), and hence it is the best method to prove that you are not dealing with a diamond. 

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  • Frequently – but not always – there are strange inclusions in Synthetic Moissanite, such as long and white needles, visible through either the pavilion or table.

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What should you do if you are not sure about the authenticity of a stone? 
If you have any doubt about a stone, a loose or mounted in a jewellery piece, make sure you have it checked by a lab like HRD Antwerp, which has all the tools, methods and trained experts needed to identify your stone with 100% certainty. 
If our extensive examination reveals your stone is a natural diamond, we can issue a grading report for your loose diamond or jewellery piece. We do not issue grading reports for diamond simulants.

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