Turnaround

Known to the Ancient Greeks as adamas ("untameable" or "unconquerable") and sometimes called adamant, diamond is the hardest known naturally occurring substance, 10 being the highest Mohs scale of mineral hardness. The material boron nitride, when in a form structurally identical to diamond, is nearly as hard as diamond; a currently hypothetical material, beta carbon nitride, may also be as hard or harder in one form. Furthermore, it has been shown that ultrahard fullerite (C60) (not to be confused with P-SWNT Fullerite) when testing diamond hardness with a scanning force microscope can scratch diamond.

In 1959 researchers determined that the majority (over 99%) of natural diamonds contain sub-microscopic nitrogen as an impurity within its crystal structure. The nitrogen atoms replace some of the carbon atoms within the structure. These nitrogen containing diamonds are termed 'Type I'. Diamonds which do not contain detectable nitrogen as an impurity are named 'Type II.'
The Swiss Gemmological Institute (SSEF), in Basel, and the University of Nantes, in France, have together developed a two-part device that easily distinguishes Type II from Type I diamonds. Its technology is based on the fact that Type II diamonds are transparent to short-wave ultraviolet (SWUV) light, whereas the vast majority of Type I diamonds block SWUV light.
A Type IIa diamond (111) has a hardness value of 167 GPa (±6) when scratched with an ultrahard fullerite tip. A Type IIa diamond (111) has a hardness value of 231 GPa (±5) when scratched with a diamond tip which leads to hypothetically inflated values.

Type II diamonds include diamonds in which nitrogen-related defects are not responsible for optical and paramagnetic absorption. Type II diamonds are divided into three subclasses:

IIa: diamonds which do not show the IR absorption band related to boron and hydrogen impurities

IIb: diamonds showing optical absorption due to boron impurities

IIc: diamonds with hydrogen-related absorption

Type IIa diamonds are often absolutely colourless and exhibit an extreme transparency. A number of large historical diamonds, such as the Cullinan and the Koh-i-Noor diamonds, are of type IIa.

D-colour diamonds, rare in nature and difficult for the lay person to distinguish from other top colourless diamond grades, may fall into one of the following groups:

Type IaA - may have very faint hint of colour with no fluorescence

Type IaAB - may have very faint hint of colour, with faint to medium blue fluorescence

Type IaB - may have very faint hint of colour, with strong to very strong blue fluorescence

Type IIa - may be absolutely colourless without any fluorescence.

Type IIa - may be a decolorised brown HPHT treated diamond.

Type IIa - may be a gem quality synthetic diamond

Type IIa diamond can be colored pink, red, or brown due to structural defects within the crystal arising through plastic deformation during crystal. Type IIb diamonds, which account for 0.1 percent of gem diamonds, are usually a steely blue or grey due to scattered boron within the crystal matrix; these diamonds are also semiconductors, unlike other diamond types. However, an overabundance of hydrogen can also impart a blue color; these are not necessarily Type IIb. Type II diamonds absorb in a different region of the infrared, and transmit in the ultraviolet below 225 nm, unlike Type I diamonds. They also have differing fluorescence characteristics, but no discernable visible absorption spectrum.

Natural type II diamonds are characterized by their irregular shapes, which are very different from the generally observed octahedral or rounded dodecahedral morphologies of Type I diamonds. Sorting of Type II rough diamonds has been empirically based on this morphological characteristic. The reason why Type II diamonds exhibit such an irregular morphology is simple. They are not as-grown crystals, but are the result of crystals fractured during their ascent in magmas. Type II diamonds are much purer with much smaller contents of nitrogen than Type I diamonds, making them plastically much weaker than Type I. When both Type I and Type II diamonds experienced similar amounts of stress while being transported from the depths to the surface of the Earth, only type II diamonds were plastically deformed and further fractured into pieces, while Type I diamonds retained their original as-grown morphology.

Type IIa diamonds can have their structural deformations "repaired" via a high-temperature, high-pressure (HTHP) process, removing much or all of the diamond's colour. The high-pressure, high-temperature (HPHT) process by which light brown type IIa diamonds can be decolourised to improve their colour grades. D colour grades can be accomplished. To identify conclusively such treated diamonds remains a challenge to laboratory gemmologists. All diamonds submitted to a testing laboratory are tested for type and any Type IIa diamonds are subjected to further rigorous scrutiny.

Before High pressure-High temperature (HPHT)-treated diamonds came to market, most gem laboratories traditionally identified Type II diamonds by infrared spectrometry, a method that can be applied in advanced laboratories but is expensive and inconvenient for use by the trade. An inexpensive, pocket-sized instrument, developed by the Swiss Gemmological Institute (SSEF), in Basel, and the University of Nantes, in France is equipped with a white fluorescent screen that glows green when illuminated with SWUV light. If the diamond being tested is Type I, the stone will block the SWUV light and the screen will remain white. When a tested diamond is Type II, the stone lets the SWUV light pass through so that the instrument’s screen glows green. This instrument can be used with either cut or rough diamonds.

The SSEF, which in April 2000 became the first laboratory to announce its ability to identify HPHT- treated diamonds scientifically, developed the instrument after testing hundreds of diamonds from the stock of Geneva’s diamantaires and finding puzzling results. Type II diamonds represent only some 1 to 3 percent of the diamonds in the total market, but 80 percent of the winter 2000 Christie’s Geneva auction offerings of D color weighing between 3.26 carats and 60.19 carats registered as Type IIa. Surprisingly, of the same lot, 100 percent of the diamonds over 10 carats were of Type II. This showed that HPHT treatment might affect not only the known 1 to 3 percent of the diamond market but also the large high-quality, high-priced market for premium diamonds.

Type IIa diamonds are the only type that can be transformed from brownish to colourless of a higher value — up to D in colour — by HPHT treatment. It has been reported that pinkish and blue Type II diamonds gain better colour by totally annealing their dominant brownish hue. However, it is apparent that the annealing process to affect the colour change is only effective for stones in excess of 1 carat, making the process expensive. HPHT diamonds are generally produced by Kaplan Lazar and are labelled as HPHT diamonds. It is well known that De Beers is not in favour of this artificial colour enhancing of Type II diamonds.

It is not possible to distinguish Type I diamonds from Type II diamonds using a jeweller's lens. Rough diamond dealers are therefore unable to determine the presence of Type II diamonds in any parcel of rough diamonds.