It’s a gemstone famously prized for its lack of colour, so it might surprise people to know that many of the world’s most-coveted coloured gemstones are also diamonds.
Formed up to 200km deep within the Earth’s mantle under extremely high pressures and temperatures, diamonds are carried to the surface in viscous, volcanic magma. Uniquely composed of a single element, diamond is typically about 99.95 per cent carbon, with the remaining 0.05 per cent incorporating one or more trace elements, often nitrogen, that can influence body colour.
Just one in 10,000 gem-quality diamonds is classified as ‘fancy-coloured’. The various colours are formed under unique conditions involving specific trace elements, radiation and/or plastic deformation of the crystal structure.
White diamonds are rated from D to Z on the industry’s accepted colour-grading system, where D is totally colourless and Z is a pale yellow or light brown colour. The term ‘fancy’ is used for coloured stones that have greater depth of colour and are graded on a different scale.
Diamonds are found in many colours, including yellow, brown, green, blue, violet, pink, red, grey and black, and even very slight colour differences significantly impact stone prices. Fancy yellow and browns are more abundant than other hues, and other colours are hard to find even in weak saturations. Rarest is the red diamond, of which only a small handful have been found.
Diamonds typically contain nitrogen as an impurity. In the case of fancy yellow to orange gems, the nitrogen atoms are singularly substituted for carbon atoms, resulting in the absorption of blue and violet wavelengths of light and producing yellow through oranges hues.
Blue to violet diamonds are the result of boron impurities; the more boron the deeper the saturation. Perhaps the most famous coloured diamond is the priceless, blue Hope diamond, which has a history of ownership dating back almost 400 years. Some blue stones may contain hydrogen as an impurity, as occurs in Australia’s Argyle mine.
For pink and purple diamonds, it is suspected that the colouration is due to defects in the atomic structure as a result of ‘plastic deformation’ during the crystal’s journey to the Earth’s surface in hot magma.
Green diamonds result from an exposure to radioactive uranium, which emits powerful gamma rays that create a deficiency by knocking individual carbon atoms out of the crystal lattice. These ‘holes’ absorb more of the blue and red wavelengths, resulting in green coloured stones. The more radiation that impacts the diamond, the deeper the green.
Radiation treatments are readily available to produce colours in diamonds, so consumers should be cautious when presented with a coloured diamond at a surprisingly modest price.
Classified according to their hue, tone and saturation, the value of coloured stones generally increases with the strength and purity of the colour. Large, fancy-coloured diamonds are extremely rare, and correspondingly valuable because they are a wonderful sight to behold.