Differentiating Diamond and its Imitation

by Erum Qureshi

There are various decisive tests to assess the authenticity of a diamond. The Ceres Diamond probe, Rayner Diamond Tester, Diamond Pen are a few popular instruments.

The unique hardness of diamond enables it to scratch the polished surface of synthetic corundum and no other substance on earth can do this. However, using hardness as a test is considered crude and seldom necessary since the diamond itself may suffer some damage in doing so.

The most notable diamond Simulants are synthetic cubic zirconia, synthetic strontium titanate, YAG (yttrium aluminium garnet) and GGG (gadolinium gallium garnet).

Synthetic white spinel, made from the Verneuil flame-fusion process is also used as a substitute for small diamonds in multiple gem settings; moreover, it is singly refractive. But it gives a Refractometer reading of 1.726, has a low degree of fire, and measures 8 on Moh’s scale of hardness. Synthetic rutile has six times the dispersion of diamond but is easily recognizable because of its high refractive indices and a large double refraction. YAG (yttrium Aluminium garnet) has the appearance of having properties of a diamond Simulants, and is differentiated from diamond by the Immersion Contrast method.

Danger of confusion between diamond and its simulants became much more apparent when Cubic Zirconia appeared on the market.

There was a demand for developing special apparatus that would make the distinction between diamond and its Simulants rapid and certain, even in the case of mounted stones and with a very rudimentary knowledge of gemology. The most ingenious and effective method of differentiating diamond from all other gemstones makes use of its outstanding property as a thermal conductor, which is higher than for any other substance – higher than even copper or silver.

The Ceres Diamond probe was the first to exploit this property even with small specimens and in the case of mounted stones. The Ceres probe has two thermistors and a small copper tip in a convenient holder. When the instrument is switched on, the tip of the probe warms up. When it is gently held to the surface of the stone to be tested, the needle of the instrument swings to the right and a green light flashes in case of a diamond. With any other stone, the needle of the meter swings to the left and a red light flashes. The probe of the Ceres instrument is very delicate and needs handling with care, but even very small stones can be tested with it.

Another instrument of the same kind is the British made Rayner Diamond Tester. Such equipment is very costly but to anyone dealing with diamond jewelry, this is a relatively small matter when compared with the value of the goods tested.

The GIA has made use of another property of diamond to differentiate it from simulants. This particular property is the diamond’s affinity for grease or greasy liquids. The Gemological Institute of America has produced a Diamond Pen, charged with a specially prepared liquid, which left a visible mark when drawn across the table facet of a diamond, but which broke up into droplets on the surface of all other stones.

Another test for singling out a diamond simulant is the Immersion Contrast method. When light is placed over stones (Strontium titanate, YAG, GGG and CZ) immersed in di-iodomethane (methylene iodide; a high density liquid with a refractive index of 1.742) or Refractometer contact fluid (RI 1.81) different stones show differing patterns. All except strontium titanate show a dark ring diminishing in width as their refractive index approaches near to that of the liquid. This is a definite visual indication of differentiation between diamond and its simulant.

Another diamond simulant, though very rare, is a diamond doublet. The top half (crown) of this consists of a diamond, which is cemented on to a pavilion of some other colorless stone. When such a stone is viewed obliquely through the table facet, a shadow of the edges of the facet can be seen on the underlying cement layer, revealing the fake.

Another way to differentiate between a diamond and its simulant is ultra violet radiation. Under long-wave ultra-violet light, diamonds will show a very varied degree of fluorescence. If all the ‘diamonds’ in a multi-stone setting show a similar fluorescence, they are certainly not diamonds. Under X-rays, almost all diamonds show a blue fluorescence and a brief exposure on film will show diamonds to be far more transparent to X-rays than any other stone. This technique is worth practicing as it is a decisive test.