Amongst the rarest and priciest of fancy coloured diamonds, they have captured the imagination of many diamond aficionados. Their colour is most often imparted by boron trapped in minute amounts in a crystal lattice otherwise free of impurities classifying them as Type IIb diamonds. The more boron is contained within, the greater the blue intensity of the diamond. Boron also makes these diamonds electrically semiconductive. Unfortunately, only a few diamonds are visible pure with most being noticeably greyish. The grey modifying hue is caused by hydrogen-related colour centres, both in Type IIa blues, which also feature more graining and plastic deformation or as in the case of Argyle Type Ia (violetish) blue diamonds where the blue is rather derived from natural radiation damage as opposed to boron, a feature of all Type I blues, modified or not. Type I Argyle blues also contain Nitrogen.
Generally the purer and deeper the blue, the rarer. Bright, pure intense vivid and deep blues which are nearly all Type IIb to violetish Blues are always exceptionally scarce and prices over the last decades have continuously escalated, particularly in 1 to 3ct sizes where they outperform pinks and greens. Given the size limitations of blue diamonds and high prices, demands exist at all saturation levels, however, the highest demand is infancy to fancy deep. Dark stones are of little interest since they are often simply too inky.
Blue diamonds come from South Africa, Central african Republic, Russia, the Old Indian Kollur/Golconda mines and Brazil. Brazilian stones are of particular note since they often appear a beautiful and highly quoted intense to vivid greenish Blue Type to Green-Blue Ia and IIa perceived as a stunning Turquoise or Aquamarine Colour which has been imparted by General Radiation centres.
According to the GIA Blue,violetish Blue, greenish Blue, Green-Blue, greyish Blue nd Grey-Blue (modified) hues are distinguished in decreasing order of value. Visually brownish Blue diamonds are known to exist, however they are very unusual in that their colour emanates from alternate graining and colour zoning, each on their own regularly seen in normal blue diamonds.
Lastly exotic bluish, orange and red (Hope diamond) phosoperescence is observed in blue diamonds, notably Type IIb’s.