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Fake Diamonds

Ever since diamonds became popular and known as the rare and expensive king of all gem stones people tried to find substitute materials to mimic their beauty and sparkle. Diamond simulants only appear similar to diamonds in their appearance (cut, color, transparency) while they do not possess a diamond´s chemical or physical properties.

The two most common diamond simulants are cubic zirkonia and synthetic moissanites which will be explained in the following in detail. The different material properties allow for proper identification of these substitutes for diamonds.

Zirconia, synthethic moissanite or diamond?




Refractive Index
Dispersion
Specific Weight
Hardness
Thermal Conductivity
Electrical Conductivity
Diamond
2,42
0,044
3,51
10
very high
none*
Synthetic Moissanite
2,65 - 2,70
0,104
3,2
9,25
high
high to low**
Cubic Zirconia
2,19
0,060
5,65
8 - 8,5
very low
very low

* < 0,1% of all natural diamonds (type IIb) contain Borum and do conduct electricity (blue diamonds)
** A new generation of synthetic moissanites show significantly less electrical conductivity (Updated: 2017)

Cubic Zirconia

Discovered by two German mineralogists (Stackelberg und Chudoba) and later produced in Sowjet laboratories cubic zirconia used to be until the most important diamond simulant until the 1990s. In the beginning these stones were yellowish, however, with heat treatment they became whiter and better so that today their completely colorless, sparkling appearance comes close to diamonds. In comparison to diamonds cubic zirconia is softer (8 instead of 10 at the Moh scale). That is why they easily get scratched and become dull. Zirconia has a higher dispersion compared to diamond which results in more "fire", i.e. the splitting of white light into the sprectral colors. As a matter of fact due to its softness zirconia stones lose this trait rather quickly - especially when set in jewelry that is worn every day. In the following pictures you can see the amount of colorful effects due to the dispersion. The lower refractive index of 2.19 causes a lesser degree of brilliance though. Light that is split cannot be returned to the eye as brilliance.



In these pictures you can see the different colors due to the high dispersion (splitting of white light in its spectral colors)


   

The lower refractive index of 2.19 ,however, leads to less brilliance.





How to identify zirconia?

The material properties make it easy to distinguish between cubic zirconia and diamond:

  • The texture of the surface often shows round facet corners and edges. Also, older stones tend to show significant scratching and wearing out of the surface area due to the softness of the material. This is easy to spot for the trained eye using a 10-x jewellers lupe
  • Specific types of inclusions can be observed in the stone that are not typical for diamonds (for example gas bubbles)
  • A hydrostatic scale determines the specific weight which identifies the material
  • Using the Scharffenberg-Formula one can calculate the average weight of a diamond based on its measurements. If the result deviates from what the carat scale says then you know you are looking at a simulant. Due to the different specific weight cubic zirkonia is about 70% heavier than a diamond with the same measurements
  • A diamond tester quickly answers the question by measuring the electrical and thermal conductivity properties



Additional methods

that can give you good hints:
  • Diamond attracts fat, yet it is water-repellent. The trained eye can see if a stone does fog up if you breath upon it. Diamonds never fog up, cubic zirkonia does.
  • The line-test: For the test to work the proportions of the round brilliant cut must be correct. This does not work on fancy cuts. If you see the line through the stone then it is not a diamond. Due to its high refractive index and the correct pattern of the facets it is not possible to "look through" a diamond. The round brilliant cut is made to throw light back in the right angle and the play of light within the stone is optimized for that purpose.


Line-Test






   
Hydrostatic Scale





Synthetic Moissanite

Moissanite (also knows as silicon carbide) is a rarely occuring mineral. Discovered in 1904 by Henri Moissan in the Canyon-Diablo-Meteor in Arizona it was at first mistaken for diamond due to its hardness and endurance in acid. The artifical production had been possible early on already but only in 1997 synthetic moissanites were produced in a quality high enough that made them suitable for the use in jewelry. The exceptional hardness, its high dispersion and refractive index made it through constantly improving its colors (less yellowish and more transparent) a dangerously-good diamond simulant. Even jewellers that back then had little experience with this new material would mistake moissanites for diamonds. Furthermore, the high thermal conductivity of moissanite is similar to diamonds which rendered this distinguishing material feature useless when trying to identify a diamond from moissanite. While the thermal conductivity allows to separate cubic zirconia (low thermal conductivity) it does not work with for moissanite.
One important characteristic trait for moissanite however is double refraction. Different refrative indexes are responsible for splitting the ray of light at the optical axis into two single rays with different directions. This leads to a "doubling"-appearance.

Unlike diamonds moissanites are not measured in carats. The diameter in mm is used to refer to their size. The different specific weight as well as slightly different alignment of facets (for optimzing brilliance based on the refractive index) is the reason that the "carat size" or weight factor does not allow for comparing diamonds and moissanites. The latter tend to be about 10% less in weight.

mm
Moissanite (average carat weight)
Diamond (carat weight)
3.0
0.09
0.10
4.0
0.22
0.23
4.5
0.29
0.33
5.0
0.41
0.50
6.0
0.68
0.80
6.5
0.88
1.00
7.0
1.10
1.20

How to identify moissanite?


Similar to distinguishing between cubic zirconia and diamond the specific material traits of moissanite allow for the exact identification of the stone:
  • Observation of specific inclusions (Small needles that look like drilling holes, no typical diamond inclusions visible)
  • Laser inscription at the girdle: Most certified diamonds (natural as well as synthetic) and moissanites created by famous brands do have a laser inscription with the company name and/or certification number
  • Using a hydrostatic scale distinguishing between moissanite and diamond is easy. The specific weight cannot be fooled
  • The Scharffenberg-Formula calculates the average round brilliant weight for diamonds based on its measurements that are defined for the round brilliant cut. Does the carat scale show a different result? Likely you are not dealing with a diamond. Moissanites weigh about 10% less than diamonds given the same measurements
  • The diamond tester answers the question efficiently: By measuring the thermal and electrical conductivity the material can be identified. Just be aware: The latest generation of moissanites shows far less electrical conductivity so that older diamond testers can be fooled into testing diamonds if their calibration is not sensitve enough 
  • The above mentioned double refraction reveals the truth: The doubling effect of the facet edges is visible with a 10-fold magnifying loupe as well as under the microscope. Important: Double refraction happens on the optical axis of the material. Moissanites are cut on purpose along this axis so that when you look into the stone directly from above into the table you will not see it. In order to see it you need to look sideways into the stone through the upper main facets towards the pavillion of the stone or from the pavillion up towards the edge of the table facet. Needle-like inclusions and the doubling effect you can see here.




 The example of calcite:
The effect of double refraction



   
The line of sight is crucial in order to see the doubling effect within moissanite stones:




Double refraction as seen in moissanite:



   
The doubling effect of the facet edges is clearly visible




Moissanite view through the table facet (no doubling effect due to the optical axis)



   
Additional hint: Needle-like inclusions are typical for moissanites




Diamond comparison: No doubling effect





   
Diamond: Sharp facet edges




Laser inscription at the girdle: GIA certificate number







   
Laser inscription at the girdle: Company name