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natural ones they are imitating and more diffi cult to identify as treated. When assessing black stones the

gemmologist has to consider all the diff erent alternatives which are available, just as for a white diamond. It is standard practice for the initial visual assessment to be carried out by an expert using a 10x magnifi cation loupe. This is obviously a less revealing observation on a dark opaque stone as it is much more diffi cult for the gemmologist to see inside it to assess it. There are a number of possible

alternatives if the stone proves not to be a diamond. The two most likely are black synthetic moissanite and cubic zirconia (CZ). Colourless synthetic moissanite is relatively easy to detect as it is doubly refractive but black moissanite and small black CZs may be used in a a melee, and a layman would never be able to detect them. In identifying black gemstones the

AnchorCert gemmologists will fi rst carry out a detailed visual assessment under magnifi cation using a strong fi bre optic light in order to see the inclusions within the black stone. The outcome of this initial assessment will determine whether the stone could possibly be diamond, moissanite or cubic zirconia. Natural black diamonds typically

diamonds are often of lower clarity and colour but have a high lustre when polished. Treatments are now more technically advanced and “graphitization”, which enhances a pale black diamond to a more sought after dark black, is induced by annealing. This treatment requires high tech equipment which subjects the diamond to temperatures of up to 1300˚C within a vacuum, but these treated black diamonds are much more similar to the

November 2013 |

contain black needle-like inclusions or irregular-shaped dark inclusions scattered throughout the stone. High temperature heat treated diamonds contain minute particles of graphite that are closely packed and appear perfectly black even under magnifi cation. By contrast black moissanite is solidly opaque and does not show fi ne graphite concentration characteristics like heat treated diamonds. moissanites’ high degree of hardness also prevents facet edges being blunt like cubic zirconia. Following

visual inspection a thermal tester can be used to distinguish

between diamonds and simulants such as

cubic zirconia, sapphire,

spinel, and glass. Diamond is a very good conductor of heat, but synthetic moissanite is still better, and normal diamond testers will identify synthetic moissanite as diamond. More specialised tests are therefore needed. The distinction between diamond

and its simulants can be clearly made by analysing elements contained in each material in fl uorescent X-ray analysis. The major element of diamond is carbon, while other simulants contain other various elements. Moissanite is composed of silicon and carbon and elemental analysis will detect silicon, which would never be present in a diamond. However, for individual

identifi cation of suspicious stones, microscopic Raman spectral analysis is essential. This technique relies upon what is known as the Raman eff ect –‘a natural phenomenon that when a monochromatic light hits a substance and is scattered, the wavelength and specifi c degree of the scattered light is unique to the substance’. This technique can be applied by

an expert to identify a substance or analyse molecular structure, and can be applied to small samples just several microns in size. It can therefore perform individual analysis quickly even on stones set in jewellery, and can positively identify diamond, synthetic moissanite or CZ. With the increasing development of synthetics and enhancement treatments for all types of diamonds, a Raman is an essential piece of equipment for any serious gemmological laboratory.

FOCUS ON THE EXPERT Anu Manchanda MSc, GG, DGA, FGA, P J Dip, F.N.A.G. Pearl Graduate (GIA) is a recipient of the Christie’s Prize for Gemmology. As ‘Senior Gemmologist’ Anu is responsible for accuracy and new initiatives in the AnchorCert diamond grading and gem testing laboratory. She is also a tutor for the Gemmological Association of Great Britain.

Jewellery Focus | 43

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