Diamonds are a natural product and their quality varies widely, which in turn affects their value significantly. The diamond trade has always sought ways to improve less valuable, potentially unsalable, diamonds by enhancing their colour or clarity. Anu Manchanda, Senior Gemmologist for AnchorCert, the diamond certification division of The Birmingham Assay Office, explains how the most common enhancements are carried out and the tell tale signs which helps gemmologists to detect them. All of the practices described below are accepted by the trade but must be disclosed so the customer knows they are buying a treated diamond and pays the appropriate price.
Diamond treatment usually involves improving the total quality of the diamond by modifying the inclusions and/or enhancing or changing the colour. Some treatments are stable and permanent while others may not be that stable and can suffer damage during setting, wear, cleaning and repair.
Cracks and fractures in a diamond reduce its clarity, but can be treated by filling. Fracture filling utilizes a glass-like material which has similar optical properties to that of diamond. It will improve the diamond’s appearance by almost two clarity grades but is not a permanent treatment. Most obvious evidence of fracture filled diamonds is bright flashes of changing colour that can be seen under proper lighting. Other evidence is presence of bubbles in the fracture of the filler or ‘crackled appearance’ of the filler. All these can be picked by 10 X magnification.
Some recent more advanced fillers may not show the colour flashes. This fracture filling can be confirmed by X-rays; diamonds are transparent to x-rays while the glass filler is not and will show an image on the photographic plate. It is general practice that gemmologists will not apply a colour or clarity grade to fracture filled diamonds.
Laser drilling is used to reduce the visibility of natural inclusions. A hole is drilled using a laser beam, with a diameter less than 0.02mm, through the diamond to reach the inclusion. Carbon-di-oxide lasers are used to heat a tiny area of the diamond until it evaporates, forming a tube reaching the inclusion. The inclusion is then vaporised with laser, bleached or etched with acids. The inclusion becomes less apparent, makes the diamond more saleable but it is still present. Laser Drilling is a permanent treatment and has become prevalent in smaller stones. It can be detected with 10 X magnification and the microscope. Sometimes laser drill channels are glass filled and they show colour flashes. Diamonds that are both laser drilled and fracture filled are not colour and clarity graded.
Internal Laser Treatment:
In Internal laser treatment there is no drill hole or drill channel present but the laser leaves a worm like squiggle; this treatment is also known as KM treatment. KM stands for ‘Kiduah Meyuhad’ in Hebrew meaning ‘special drill’. This treatment is used on black inclusions which are accompanied by internal fractures. One or more pulses of the laser beam is focused on the inclusion with the resulting heat creating an internal fracture. The inclusion is then bleached by boiling the diamond in strong acid under pressure. The created feather is very difficult to differentiate from the natural feather; but it can be easily spotted under high magnification of a microscope using different lighting techniques by an experienced diamond grader/gemmologist.
Surface coating: Surface coating applies a thin layer of coloured foreign material to all or part of a diamond surface, to either mask the underlying body colour or enhance a desirable colour. Most often, this coating is applied to the pavilion and/or girdle of the diamond; the way that light refracts as it passes through a diamond then creates the illusion of uniform colour distribution. The durability of diamond coatings vary considerably, depending on materials used and methods of coating applied. Most recent advances in technology employ a very thin optical or chemical film which is more durable than older methods, but still readily worn away by heat, scratching, abrasion, polishing, and just everyday wear. The coating, if damaged; can be picked up under microscope by an experienced diamond grader/gemmologist.
High Pressure High Temperature Treatment (HPHT):
HPHT treatment is mostly applied to Type IIa diamonds which have no nitrogen and are usually colourless but due to distortion of crystal lattice sometimes show brown colour. Exposing the diamond to high pressure and high temperature eliminates the brown colour by reducing or removing the structural irregularities in the crystal lattice.
In Type Ia diamonds, of brown colour, the HPHT process causes dispersion of paired nitrogen into single nitrogen molecules and causes the diamond to take on a fancy yellow to yellow-green colour. This transforms an unsalable poor colour diamond into a valuable fancy colour diamond but this is usually easily detected as the stone will have a a strong reaction to ultra violet light that helps to identify the HPHT treated fancy yellow-green diamond. The HPHT treatment is detected by testing the Photoluminescence using Raman spectroscope under liquid nitrogen temperature (-196°C)
Irradiation & Annealing: Radiation from high energy particles is used in creating colour centres in diamonds. These radiations alter or damage the crystal structure and change or intensify colour. The most common colours caused by irradiation are different hues of blue and green. Heating (annealing) the diamond after bombarding it with charged particles results in the modification of the bluish green colours to yellows, oranges and sometimes pinks, reds and purples. Depending on the type of radiation used the colour may be restricted to the surface or penetrate the whole stone. This is a stable treatment under normal situations. The treatment can be detected by testing the absorption spectrum using UV-Vis Spectrophotometer and Photoluminescence using Raman spectroscope under liquid nitrogen temperature.
Heat Treatment, Graphite Treatment: Low quality, heavily included diamonds are heated in a vacuum to form graphite deposits within the fractures, resulting in black diamonds. The majority of black diamonds found in the diamond market are treated and spectroscopic methods are used to authenticate the origin of colour. Multiple treatments on diamonds have become common. AnchorCert gemmologists see examples of these treatments on a regular basis and use their sophisticated equipment to ensure all treatments are disclosed as required.
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