If one observes certain characteristic properties of moissanite one can easily identify a moissanite from a diamond. But one must admit that in the present times is a very good simulant for diamond and has excellent properties. In fact, as seen from the results of heating of diamond and moissanite in the presence of oxygen, the diamond burns off but moissanite remains. So is the diamond really for ever?
(Dr.Jayshree Panjikar is the Chief Mentor & Strategic Advisor SeamEdu’s Institute of Gem & Jewellery, Pune-India)
in a Furnace presence of Oxygen Moissanite remains
When a carat sized moissanite is placed in a small dish of water and a bright light is focused on it, then the higher refractive index is clearly displayed by the increased length of the light specks and the larger size of the ring of specks emerging from the moissanite.
The very high fire or dispersion (0.140) in Moissanite is clearly displayed by the deep, intense spectrum of color associated with each speck. Whereas, if one-carat diamond in water is exposed to a high intensity light shining on it the fire (dispersion) displayed by the spectrum of color associated with each speck is much less as compared to moissanite.
When observed under microscope a moissanite shows two important characteristics. The lower pavilion facet edges appear with double lines. With the result one can easily identify a moissanite with a 10x lens. Secondly a moissanite also shows typical needle like inclusions. These needles are also very typical for moissanite can also be seen with the loupe.
At the SeamEdu’s Institute of Gem & Jewellery, in Pune, students are taught easy methods to identify moissanite without using many sophisticated instruments.
One sees rainbow colours through diamond, cubic zirconia and moissanite
Rainbow colours or spectral colours are seen in a gem because of the Dispersive Power of the stone. Every gemstone has the power to disperse or break white light into seven colours. Diamond has dispersive power of 0.044, therefore it shows very good fire in a faceted stone. The dispersive power of cubic zirconia is 0.60 therefore it shows more flashes of colour than diamond does. Moissanite has very high dispersive power of 0.104, which is 2.4 times more, hence the Fire or flashes of colour seen in moissanite are much more than those seen in diamond.
Diamond has adamantine luster. It is the top quality of luster among gemstones. Mostly gems have vitreous luster but diamond is far superior. There are very few gems which can compare in luster with diamond and even these have sub adamantine luster like cubic Zirconia, zircon, demantoid garnet. Moissanite has also Sub Adamantine luster.
How is Moissanite produced?
Moissanite is grown using a process called Sublimation, a process in which a solid can become gas without passing through the liquid state. As well as gas can directly solidify to form crystals without passing through the liquid state.
Why do ordinary diamond testers identify moissanite as diamond?
Diamond is a very good conductor of heat. It conducts heat at the rate of 0.55 - 1.7calories/cm per degree Celsius whereas Moissanite is still better conductor of heat and conducts heat at the rate of 1.6 - 4.8 calories/cm per degree Celsius. As there is an overlap of the thermal inertia ranges of diamond and moissanite between 1.6 to 1.7calories, the normal diamond tester would identify moissanite as diamond.
Therefore to detect moissanite a special moissanite tester is required.
The famous scientist, Dr.Henri Moissan who was analyzing a meteorite, found natural silicon carbide in it, therefore in his honor the name “Moissanite” was given to it. And in 1986 Dr. Moore found that Natural Moissanite crystals occur as inclusions in diamond crystals.
Diamond Identification was easy… Not any more
One of the easiest tests for the identification of rough diamonds had been the scratch test. In fact many diamond diggers keep a small plate of synthetic corundum or synthetic ruby to check whether the stone they have found is diamond. Diamond has a hardness of 10 on the Mohs’ scale of hardness. Whereas Corundum has hardness of 9 hence diamond can scratch the corundum plate. So any white stone that scratched the ruby plate was considered to be diamond.
But Moissanite has hardness of 91/4 hence it too scratches corundum. Hence all materials which scratch corundum need not be diamond!
Just for information the Hardness on Knoop’s scale, diamond can with stand a pressure of 8000kg/mm2, whereas Corundum can bear 2000kg/mm2 and Moissanite can tolerate a pressure of 2954kg/mm2.
Moissanite is crystalline silicon carbide - SiC. It is a man made transparent crystal which if faceted properly can look very much like a diamond! If the moissanite is set in a piece of jewellery and is then tested with a diamond tester, it would be wrongly detected as diamond.
In 1893, Dr. Acheson was trying to make synthetic diamonds, but his process had some contamination and therefore he landed up making silicon carbide accidentally. This material was called “carborundum”. It is a very good abrasive. So moissanite is the transparent carborundum.
Man Made GEM - Moissanite
Dr. Jayshree Panjikar
Crystals of Moissanite are produced by using this process where powdered silicon carbide is heated in a double cylinder to vapour stage. Then this vapour of silicon carbide is passed through the inner graphite cylinder. Growth takes place directly from vapour phase on a seed crystal without passing through a liquid phase.
It is reported that it takes 6hours to grow a crystal of 12mm diameter and 6mm thickness.
Moissanite is polymorphous
Moissanite can occur in different forms. This property is called polymorphism. Diamond and graphite are polymorphs of carbon. Both have same chemical composition but the structure and arrangement atoms are different. In diamond the arrangement of atoms is cubic whereas in graphite the atoms are arranged in a hexagonal manner.
In the case of moissanite, depending upon the stacking sequence of hexagonal layers of silicon carbide different polymorphs or poly-types of moissanite have been identified.(6H or 4H).Therefore it is also possible to have cubic, hexagonal, rhombohedral crystal structures for moissanite.
At present only Alpha-SiC, hexagonal (6H and 4H polytypes) moissanite is grown. Beta-SiC is cubic and has crystal structure similar to that of diamond, but at present it can not be grown in bulk. Silicon carbide grown in Cubic structure will be more difficult to identify.
Moissanite crystallizes in Hexagonal Structure
Moissanite crystallizes in hexagonal system and is uniaxial positive.
Whereas diamond crystallizes in the cubic system accordingly its properties are different. If one considers just light transmission in a diamond, then through a diamond a ray of light is transmitted as a single ray i.e. there is single refraction.
But in the case of moissanite since it crystallizes in the hexagonal system it shows double refraction. The one ray of light which enters the moissanite is spilt into two rays, one ordinary ray and the other extraordinary ray.
Diamond shows single refraction and therefore it is isotropic. Whereas Moissanite splits a transmitted ray and shows Double Refraction is thus Anisotropic. Therefore a loose moissanite can be tested even by a polariscope.
If one has two Polaroid filters (which are used for cameras) in crossed position, then between these filters a moissanite would appear bright four times in a complete rotation. A diamond on the other hand would appear dark throughout the rotation.
Diamond has a single refractive index
A diamond has a single refractive index of 2.42 with no birefringence whereas the moissanite has two refractive indices one for the ordinary ray 2.648 and the other for the extraordinary ray 2.691 and the difference or birefringence being 0.043.
No Read-through Effect in Diamonds and in Moissanite
When a faceted white quartz or white sapphire is placed table side down on printed paper, one can read the script through the pavilion side of the stone. This is called the Read-Through Effect. The critical angle of white sapphire is 34o36’and that for quartz is 40 o11’. Generally when the critical angle of a gem is less than 30o then there is no read through effect.
Diamond has a very low Critical angle =24o 26’. Therefore faceted diamond shows excellent Brilliance or Life as all light entering a faceted brilliant cut diamond returns to the observer. So if a faceted diamond is placed with table down on a piece of printed paper one can not read the script on the paper through the diamond.
Therefore in Moissanite the Critical angle is 22 o 10’, there is excellent Brilliance and like diamond No Read-Through Effect.
A faceted white sapphire is placed with table down
A faceted diamond is placed with table down
How to separate out a mixed packet of small sized diamond and moissanite
Diamond has a specific gravity: 3.52, therefore it sinks in Methylene iodide liquid which has SG=3.32, whereas Moissanite has specific gravity: 3.22, therefore it would float in the pure Methylene iodide. So the entire packet could be emptied in a beaker of Methylene iodide, and stones floating would be Moissanite.
Diagram Moissanite floats and diamond sinks in Methylene iodide
Effect of heat on diamond
When diamond and moissanite are heated in a furnace in presence of air/ oxygen then at 1100 deg C diamond burns off whereas moissanite remains intact.
When Diamond & Moissanite are heated…..
