Diamond Simulants- Moissanite to Sapphire..
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Gems/Natural Synthetics/Artificial Synthetics /Imitations/Composites

Sapphire/ Topaz/ Zircon/ Synthetic Rutile/ YAG/ Cubic Zirconia / Moissanite

A diamond expert is able to discern a loose diamond from a simulant just by eye alone, and is unconcerned with the following tests to separate the various impersonators, but we have included some of the differences for interest. Many jewellers use some of the tests for their own piece of mind, and no doubt you will already have seen a jeweller using one of the many diamond testers available on the market, the most common being the thermal tester. This is however unable to distinguish between diamond and Moissanite, and can catch the inexperienced appraiser.

Simulants

Diamond simulant has the same meaning as imitation and refers to any material that has the appearance of diamond but does not posess the characteristics, atomic structure, chemical composition or physical properties of natural diamond. Simulants commonly include Cubic Zirconia, Colour less Synthetic Corundum (sapphire), Synthetic Spinel, Strontium Titanate, Yttrium Aluminium Garnet (YAG), Gadolinium Gallium Garnet (GGG), and Glass (Paste).

We can split simulants into five further sub categories:

Gems
These are natural stones having the appearance of diamond. Stones include Sapphire, topaz, Beryl, Quartz, Zircon; all in their colourless forms.

Natural Synthetics
These are manufactured stones that have the same chemical composition, atomic structure, and physical properties of a natural counterpart. They are man made and include: Synthetic Rutile, Synthetic Sapphire, and Synthetic Spinel. The term cultured is starting to be applied to synthetics, but we will stay with the more correct synthetic term. Synthetic Diamond

Artificial Synthetics
These are the manufactured stones that have no natural counterpart and include: Strontium Titanate, Yttrium Aluminate (YAG) and Cubic Zirconia.

Imitations
Glasses of differing densities, often referred to a paste.

Composites
Stones were often constructed from more than one stone to enhance particular properties. Glass for example is very soft and to prevent the surface from scratching a layer of harder material was bonded to the surface. These stones are often referred to a doublets.

The Main Simulants:

White Sapphire

White Sapphire: Aluminium Oxide (alumina) Al2O3; crystal system: Trigonal; from the Indian Sanskrit Kuruvinda meaning hard stone; Hardness:9 ; Relative Density 3.99-4.05; cleavage: none; refractive Index 1.765-1.773; Birefringence: 0.008 ; Dispersion: Weak.

This is often used as a diamond substitute in cheaper jewellery without intent to deceive. Usually found in round brilliant cut form surrounding larger coloured stones in rings. A refractometer test will immediately establish its identity if the table facets are unobstructed. Also sapphire is doubly refracting and will show four extinction positions when rotated between crossed polarized filters. Visually lacking in life compared to diamond.

White Topaz

White Topaz: Hydrous Aluminium Fluorosilicate, AL2[(f,OH)2|SiO4] ; Crystal System: Orthorhombic; name derived from Sanskrit tapas meaning fire ; Relative Density: 3.49-3.57; Hardness:8; Cleavage: eminent along base; Lustre: vitreous; Refractive Index: 1.606-1.638; Birefringence:0.01; Dispersion: weak; Pleochroism:distinct.

Occasionally found in jewellery eminating from the far East and easily distinguished again by refractometer. Visually there is much less brilliance and fire than diamond. Doubly refracting, topaz is separated from diamond by rotating it between crossed nicols in a polariscope.

Zircon

White Zircon: Zirconium Silicate Zr[Si04]; crystal system: Trigonal; name derived from the Persian Zargun meaning golden; Relative Density: 4.6-4.7; Hardness: 7.5; Cleavage: Poor; Tenacity: Brittle; Refractive Index: 1.923-2.015; Birefringence 0.058; Dispersion:Strong; Pleochroism: Weak.

This is a good simulant for diamond, it is hard has a high refractive index, and good dispersion. It is visually lacking in life by comparison to diamond, but has more than both Topaz and Sapphire. However it is easy to separate with an eye glass, having a large double refraction. Viewing a back facet through the table with a lens will reveal doubling of the edges. Zircon has a distinctive appearance which is easily recognisable once noticed.

Synthetic Rutile

Synthetic Rutile: Introduced in 1948. Has a high degree of brilliance. Faint yellow colour, doubling of back facets caused by high double refraction (Birefringence: 0.287). High Relative density 4.25; Refractive Index: 2.62-2.90; Scratches easily, Hardness:6.

This looks similar to a diamond of low colour grade. It has a high refractive index and a dispersion 6.5 times that of diamond which gives it an exceptional display of fire, and far too much to be diamond. Unlike diamond, Synthetic Rutile exhibits doubling of the back facets. Even with a bluish film added to the stone, some yellow body colour always remains.

YAG

YAG: Yttroaluminate Y3Al5O12; (not related to garnet); Cubic symmetry; Relative Density: 4.57-6.69 heavy; Small Dispersion:0.028; Refractive Index: 1.83; Good Hardness:8.5; Grown by flux melt method and used widely in the early 1970's to be replaced later by Cubic Zirconia.

This was first marketed in 1969, being made famous when Richard Burton had a copy of a 69 carat pear shaped diamond given to his wife, Elizabeth Taylor, made in YAG. It is quite hard, loups clean, and is singly refractive like diamond. It has less fire than diamond, but when cut with slightly different facet angles, starts to approach a good diamond look-a-like. These newly cut stones were marketed under the name 'Diamonaire'. It is relatively hard and holds a good polish with an adamantine lustre. It has a high relative density, and when weighed is clearly too heavy for an equivalent cut in diamond. A 1.00 carat stone measured on a Moe's gauge would weigh around 1.30cts on a balance. An immersion test in refractive liquids, will reveal its lower refractive index.

Cubic Zirconia

Cubic Zirconia: Zr02 mixed with Yttrium Oxide (Y203) or Calcium Oxide (CaO) as a stabilizing agent. Refractive Index:2.16; Relative Density:6; Dispersion:0.060; Hardness: 8.5. Introduced in 1977, the similarity with the figures for diamond make this the second best simulant produced to date, second only to a new type of stone called Moissanite

Cubic Zirconia is an artificial synthetic because the stabilizing oxides used in its manufacture remain in the finished product and its structure is different to the natural material Baddeleyite Zr02. This has been such a good imitation that cubic zirconia can be purchased in both graded colours and clarities. Its refractive index is near enough for the stone to remain quite brilliant when tilted, and although the dispersion is higher than that of diamond, is not so obvious to the layman, and close enough to deceive. The relative density is much higher than diamond and a loose stone will appear to weigh too much. If set, it is easy to destructively test the stone with a diamond hardness pen.

Moissanite

Moissanite is a new product but its origins go back to 1893 when Dr. Henri Moissan, a French chemist and later a Nobel Prize winner, identified minute quantities of natural silicon carbide, known as lusenite, in the Diablo Canyon meteorite found in a remote area of Arizona. Recently, these elements were replicated in a synthetic material to produce a synthetic gemstone with similar properties to diamond. The gemstone is more brilliant than diamond, Refractive Index 2.65-2.69, has a higher lustre than diamond at 20.4%, and a hardness of 9.25 , diamond being 10. Relative Density: 3.21. Because moissanite replicates a diamond so well, it could be sold as a diamond to the unwary buyer.

Properties
  Refractive Index (Brilliance) Dispersion (Fire)
Luster
Mohs Hardness
Toughness
Specific Gravity
Synthetic Moissanite
2.65 - 2.69
0.104
20.4%
9.25
Excellent
3.21
Diamond
2.42
0.044
17.2%
10
Good
3.52
Cubic Zirconia
2.17
0.066
N/A
8.25
Good
5.80
Sapphire
1.77
0.018
7.4%
9
Excellent
4.00
Emerald
1.58
0.014
4.8%
7.5
Good to Poor
2.72

Synthetic Diamond

Synthetic diamond is any manufactured product which has the same chemical composition, atomic structure, and physical properties as natural diamond. These are manufactured in the laboratory and are currently very rare in gem quality, but supplied in high volume as industrial diamonds for use in cutting tools and more recently as heat sinks for the semiconducter industry.

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