Coloured diamonds are found both in nature and in laboratory settings. According to the GIA, only one in 10,000 natural diamonds is a coloured diamond outside the D-to-Z grading spectrum. Coloured diamonds can be found in almost any shade of the rainbow. The GIA lists 27 different hues for natural coloured diamonds. The variety of shades available has allowed jewellers to match colour names with what’s in vogue, much like the fashion industry does when introducing “new” colours: champagne (brown), pumpkin (orange), canary (yellow), etc.
Even a one-carat diamond requires billions of carbon atoms to bond, and all of those atoms must be carbon to create a colourless diamond. The slightest quirk creates a coloured diamond: a bit of boron makes a blue diamond; nitrogen makes a yellow diamond; natural radiation form nearby rocks trap electrons to create a green surface colour; pink or red shades are thought to be due to changes to the electron structure during the voyage to the surface.
Laboratories can create coloured diamonds by mimicking the tricks nature plays. Laboratories can also perform colour treatments on diamonds to enhance their colour. A slightly yellow diamond needs a thin coat of blue to neutralize the yellow and give the perception of a higher colour grade. Irradiation can turn a diamond more yellow, brown, blue, green, pink, or red.
The GIA can issue colour grades. It describes the colour of fancy diamonds in three ways: pure spectral colours of the diamond (hue); lightness and darkness of the hue (tone); and the colour’s strength and purity on a scale from neutral to vivid (saturation). Natural radiation and pressure can intensify the diamond’s colour.
Red is the rarest diamond colour and the most expensive diamond colour. Red shades are thought to be due to changes to the electron structure during the diamond’s voyage to the surface.
Nitrogen is suspected of being the modifier for orange diamonds.
Yellow diamonds are very common, but the most vivid yellow diamonds are mined in South Africa. The yellow colour comes from inclusion of nitrogen; nitrogen atoms are similar enough to carbon atoms that they easily take carbon’s place in the formation process. Throughout history, yellow diamonds that were not vivid enough to be called fancy yellow diamonds have been “enhanced” with blue colouring, in hopes of passing the diamonds as colourless.
Natural green diamonds are very rare, with an estimated less than ten coming to market each year. While some green diamonds have a trace amount of nickel mixed into their carbon, the coloration in most green diamonds is due to the natural radiation from nearby rocks, which traps electrons to create a green surface colour. Since the coloured portion is the outer layer, some of the natural green is lost during polishing.
Boron is another element that is similar in size to carbon and can easily be found in trace amounts in a diamond. However, when boron is present and the level of nitrogen is very low, the boron gifts the diamond a blue colour. Other blue diamonds are unrelated to boron; nickel or high concentrations of hydrogen are the hypothetical causes of blue colour in some diamonds. The most notable mine for blue diamonds is the Cullinan, in Pretoria, South Africa.
Pure purple is the second rarest colour of diamond. While there are several theories on why some diamonds exhibit a purple color, many scientists believe natural purple diamonds are formed due to post-growth plastic deformation while traveling from the earth’s mantle to its surface via magma. Natural purple diamonds account for 1% of the diamond mined in Siberia’s Mir kimberlite field. Most are only a pale purple, but some show a strong purple coloration. Professionals can tell a natural purple diamond from a treated purple diamond by examining the distribution of the colour. Treated stones are saturated with colour throughout; natural purple diamonds have a concentration of colour along the deformation lamellae.
Pink diamond shades are thought to be due to changes to the electron structure, called “plastic deformation” during the voyage to the surface. Pink diamonds are associated with the Argyle mine in northwestern Australia, which produces an estimated 90% of all natural pink diamonds in the world. However, pink diamonds have also been found in Brazil, Tanzania, and India.
Trace elements such as boron in the carbon structure can create a grey diamond. However, a grey diamond may also be a black diamond that is not thoroughly saturated.
Structural defects in the diamond lattice give brown diamonds their colour, as the defects absorb light. The presence of nitrogen can also give a diamond a brown hue. The terms “champagne diamond” (lightly tinted brown diamond) and “cognac diamond” (darker brown diamond) were coined to better promote these loose brown diamonds.
Brown diamonds are found in Australia, Africa, and Siberia.
Contrary to how most coloured diamonds are formed, a black diamond’s colour is not related to trace elements. Small inclusions of graphite and iron clusters create the “black” colour. While other coloured diamonds are transparent, the many inclusions in a black diamond means it is typically opaque. Black diamonds will not exhibit the fire and brilliance of a white diamond or transparent coloured diamond.
Since a black diamond necessarily has inclusions, it fractures more easily than some other gem-quality diamonds. They do not cleave as predictably as most diamonds. The difficulties involved with cutting and polishing black diamonds precludes many from becoming gem-quality, and most are declared industrial-use only.
Black diamonds found in nature usually are not consistently black. They may contain clear, white, or gray pieces. To enhance the black colour, the diamonds are subject to high-pressure, high-temperature treatment (HPHT). This is meant to darken the lighter bits of the diamond. Other methods used include dyeing the diamond or irradiation.
Perhaps this helps better define a black diamond ski slope, too. The snow may look white, but there can be hidden “inclusions” that make the path more difficult to navigate safely than other grades of ski slopes.