Diamonds are found in almost every colour of the rainbow; most are slightly yellow or brown. However, a very small percentage are other colours, such as pink, intense yellow, green, red or blue. Blue is one of the rarest and most valuable colours.

To understand why they are so rare, we need to look at the chemistry of diamonds. 98% of all diamonds are what as know as Type I ("type one"). Type I diamonds contain trace amounts of nitrogen. The other 2% of all diamonds are called "Type II" ("type two").

There is a further division of Type II diamonds, into Type IIa and Type IIb.

Type IIa are almost totally chemically pure, containing only carbon. Type IIb contain minute traces of boron.

Type IIb is the category in which we find almost all blue diamonds. Unlike the other categories, Type two diamonds are electrical semiconductors.

The boron present in Type IIb diamonds can in some cases make a diamond blue. The mechanism is not fully understood, but we do know that almost every blue diamond in the world is Type IIb. However, the chemical rarity of these diamonds means that overall, less than 0.0001% of the diamonds in the world are blue diamonds.

The colour of blue diamonds runs from very pale blue, to a rich, deep intense blue. The deeper and richer the blue the more desirable the stone is.

It is possible to colour diamonds artifically nowadays, however these diamonds are much less valuable than their natural counterparts. The majority of blue diamonds for sale in the stores are either synthetic/simulant or are natural stones that have been artificially enhanced. Such enhancement may involve irradiation and/or heat treatment.

In October 2007, a 6 carat natural blue diamond broke all previous records, becoming the most expensive diamond ever. It sold for almost 8 million dollars, or 1.3 million dollars per carat. It is pictured below. What made it so valuable was the richness of blue, and the fact that it was evenly spread across the whole of the diamond.

Any diamond with even a slight amount of blue colour is classified as a blue diamond. Many have a grey modifier, that is to say the blue is "mixed" slightly with grey. The rarity and value of a blue increases dramatically the stronger and purer the saturation of blue gets and the less grey mixed into the blue.

The most important factor in evaluating a fancy color diamond is the richness and beauty of the color. The GIA grading scale for blue diamonds ranges from "Faint" and "Very Light," to "Fancy Deep" and "Fancy Vivid" where the saturation and intensity of the color just reaches out and grabs you. Other considerations, including clarity and carat, are of much less importance than colour when discussing blue diamonds.

The most famous blue diamond is the world is the Hope Diamond. A wonderful 45 carat rich blue diamond, currently in the Smithsonian Institue in Washington. Legend has it that it is cursed, though that is a subject of another Hub some day!!