Black diamonds are no longer the reserve of mourning jewellery, and have found their niche in modern tastes. To make sure they’re genuine, senior gemmologist ANU MANCHANDA explains what she is looking for when examining these stones.
Fashion trends come and go but classic monochrome is always in vogue. This is certainly the case at the moment for gemset jewellery, where alongside white metal and glittering white stones, black diamonds and other shiny black stones have increased in popularity over the past few years. Black diamonds have moved on from their long association with mourning jewellery and are now trendy and sophisticated. Where top end designers lead, others will always follow and there are now a multitude of different black stones being used in both the fine and costume jewellery industry. As ever these are of varying quality and value and the gemmologists at AnchorCert have found themselves assessing and authenticating an increasing number of black stones.
For those who do not wish to use diamonds there are many other black coloured gemstones such as black spinel, black sapphire, synthetic cubic zirconia, black tourmaline and black onyx. Present day black synthetic moissanite is becoming very popular, gleaming Whitby jet will always have a following and then of course there is black glass. Anu says: “These diamonds may not always be truly ‘black’ when closely examined under magnification and high intensity lighting. They often include a mixture of shades of grey, brown, green, blue or violet. The black colour appearance is created by a high concentration of dark inclusions, which create a general translucency or opacity. Diamonds that contain a dark shade of brown, green, blue, violet or olive-type colour can also appear black under every day lighting.”
There are various different types of natural black diamonds. The colour can be due to a high concentration of tiny, dark particles throughout all or the majority of the stone. This will make the overall appearance a neutral grey to deep black depending upon the concentration. Alternatively the black appearance may be caused by a high concentration of dark foreign material and dark internal cleavage and micro-fissures.
Not all diamonds are single crystals. Some may be a concentration of numerous tiny diamond crystals. This form of diamond, known as Polycrystalline Bort, is commonly used for industrial purposes and is now being faceted for use in jewellery as it is naturally truly black and opaque. Another unique polycrystalline variety is carbanado. Due to its polycrystalline structure carbanado is very hard to polish and is the toughest form of diamond. The colour can range from gunmetal grey, to black, to dark brown.
Treated black diamonds enhanced by irradiation have been on the market since the 1950s. These 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 natural ones they are imitating and more diffi cult to identify as treated.
When assessing black stones the gemmologist has to consider all the different 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 magnification loupe. This is obviously a less revealing observation on a dark opaque stone as it is much more difficult 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 magnification 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 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 fluorescent 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 identification of suspicious stones, microscopic Raman spectral analysis is essential. This technique relies upon what is known as the Raman effect –‘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.
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