The rarity of natural blue diamonds is undisputed; many jewellery designers never encounter a natural example of the stone throughout their entire careers. (While it is often claimed that pink diamonds are more rare than blue, this may be due to an increased demand, rather than reduced supply.)
The only mines sourcing blue diamonds are the Cullinan mine in South Africa, the Argyle mine in Australia (soon to close) and the Golconda mine in India. This extreme rarity explains both the expense of such gems, as well as the reason why there is so comparatively little knowledge of them.
What is known, is that blue diamonds derive their noticeable blue tone from the interaction of boron molecules within the structure of the pure carbon crystal. The concentration of boron molecules in the diamond directly impacts the tone and saturation of the blue hues, ranging from from faint, to deep, dark blue.
They are virtually pure on an atomic scale with no measurable quantities of nitrogen atoms, but with a small concentration of boron atoms (less than one part per million). The colour will depend on the number of boron atoms within the stone. Most will have a secondary hue that will cause the stone to appear more green, violet or grey.
Types of diamond
A pure diamond would be composed entirely of carbon. However, diamond crystals often contain some amount of trace elements that take the place of some of the carbon atoms, the most common being nitrogen.
Diamonds can be classified into two types, depending upon whether nitrogen is significantly present or not in its carbon crystal structure.
Type I diamonds contain nitrogen atoms as a major impurity (up to 0.2%).
Type II diamonds do not contain nitrogen as a significant impurity.
Type IIb atomic structure
Type IIb diamonds contain atoms of the element boron as the major impurity. In natural type IIb diamonds, the boron concentration is only about 0.5 parts per million (0.5 ppm). The boron occurs as isolated atoms replacing carbon atoms randomly throughout the diamond crystal structure.
We look forward to offering an impressive coloured diamond ring featuring a central pear cut blue diamond weighing 0.55cts as Lot 182 of our forthcoming Fine Jewels & Timepieces auction.
– Julie Foster, Head of Fine Jewels & Timepieces