The Magic of Coloured Diamonds
Coloured diamonds are in the spotlight. They’re routinely breaking records at auction houses. They make headlines when set in celebrity engagement rings. And they’re gemmological marvels of the rarest order.
Seen in practically every colour of the rainbow, coloured diamonds are far rarer than diamonds in the D-to-Z colour range. Only one out of 10,000 carats of fashioned diamonds displays fancy colour, and a diamond’s chances of displaying intense colour are even less – one in 25,000. Browns and yellows are the most common fancy colours. Red, green, purple and orange are the rarest.
GIA® issued its first origin-of-colour reports for coloured diamonds in 1956. Backed by decades of research and the examination of thousands of coloured diamonds, the GIA Coloured Diamond Grading System has become the standard for evaluating these extremely rare gems. Current GIA Coloured Diamond Grading Reports describe colour grades using these terms: Faint, Very Light, Light, Fancy Light, Fancy, Fancy Dark, Fancy Intense, Fancy Deep and Fancy Vivid. Virtually every coloured diamond sold at major auction houses has been graded by GIA.
“The GIA Coloured Diamond Grading System allows a much more detailed description of diamond colour than what had been available under the previous grading system. This has greatly facilitated the trading of such stones worldwide, particularly as new sources expanded the quantities of stones in the market and the range of colours,” observed Russ Shor, senior industry analyst at GIA.
GIA researchers theorise that the colour in many pink diamonds is caused by colour centres that can selectively absorb light in the visible region of the spectrum. Colour centres are the result of lattice defects, or imperfections in the arrangement of atoms in a crystal. These defects can sometimes cause pink graining in the diamond crystal.
Most naturally coloured black diamonds get their colour from large quantities or clouds of minute mineral inclusions such as graphite, pyrite or haematite that extend throughout the stone. These diamonds may also have numerous cleavages or fractures that are stained black or have become black because of graphitisation. Concentrations of these internal features are responsible for the colouration. However, the actual body colour of a natural black diamond may range from near-colourless to brown or “olive” green.
The presence of nitrogen causes a diamond to appear yellow. The intensity of the yellow colour is dependent upon varying amounts of nitrogen. Yellow diamonds are considered to be a coloured diamond and graded as “Fancy” when they fall outside the D-to-Z range (colourless to light-yellow).
Once relegated to industrial use, brown diamonds gained cachet in the 1980s when marketers gave them romantic names like champagne, cognac and chocolate. The colour in natural brown diamonds is caused by internal parallel brown grain lines due to distortion of the crystal lattice (arrangement of molecules). Brown diamonds are generally more affordable compared to other fancy colours.
The presence of boron impurities is often responsible for the colour of natural blue diamonds. The more boron, the deeper the blue. However, their colour can also be caused by radiation exposure or associated with hydrogen. India was historically the source for blue diamonds. Within the last several years, notable blue diamonds have been found at the Cullinan Mine in South Africa.
Only a handful of red diamonds larger than 5 carats have been found. Red diamonds are so rare that from 1957 to 1987 GIA did not issue a lab report for a diamond with “red” as the only descriptive term. “Predominantly red” means that red is the primary colour with no secondary hues (like purple). GIA researchers, who have been studying diamonds for decades and have access to the most sophisticated equipment, are still not sure what causes their colour.
Green diamonds get their colour when radiation displaces carbon atoms from their normal positions in the crystal structure. This can happen naturally when diamond deposits lie near radioactive rocks, or artificially as a result of treatment by irradiation. Naturally coloured green diamonds are extremely rare. Because of their rarity and the very real possibility of treatment, green diamonds are always regarded with suspicion and examined carefully in gemmological laboratories. Even so, advanced gemmological testing can’t always determine colour origin in green diamonds. Examined by GIA Researchers in 1988, the 41 ct Dresden Green diamond is the largest, and perhaps the finest, green diamond known to have a colour of natural origin.
Synthetic Coloured Diamonds
In recent years, gem-quality synthetic coloured diamonds have come to market. A Russian company, New Diamond Technology, produced a 10.08 carat diamond with a colour grade equivalent to Fancy Deep blue, which GIA examined in September 2016. GIA also offers Synthetic Coloured Diamond Grading Reports for synthetic coloured diamonds.
Coloured diamonds are exceedingly rare and exceptionally beautiful. Because of this, interest in them is not likely to wane. Expect these rarities of the highest order to continue to captivate gem aficionados and the public.