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Simulants
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 the new
simulant Moissanite,
and can occasionally fool the inexperienced appraiser.
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:
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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.
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 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.
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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.
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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.
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 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.
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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.
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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 |
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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 |
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| 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. |
Production of Diamond by Chemical Vapour Deposition
(CVD)
In the 1980s reports emerged from Russia suggesting that
diamond could be formed from the vapour phase at relatively low temperatures
and pressures. Although greeted at first with some scepticism, these reports
generated a large amount of research into chemical vapour deposition (CVD)
of polycrystalline diamond. Growth of Polycrystalline Diamond
Polycrystalline diamond
can be grown by a number of different CVD techniques, including microwave
plasma assisted chemical vapour deposition, hot filament deposition, plasma
jet deposition and combustion flame deposition.
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Gem quality is now on the market in the UK, and the first synthetic diamond
sold as a real (natural) stone has been spotted in a UK laboratory. The
1.27 carat intense yellow diamond was set in a solitaire ring and purchased
in Birmingham 2000. Both customer and retailer were unaware of its nature.
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