Blue diamonds are some of the most rare and most beautiful of all the various colours of diamond. The most highly prized, deepest and purest blue diamonds are the most valuable.

Most blue diamonds are relatively small, the price per carat goes up very rapidly as the size of the diamond increases.

Blue diamonds were first found in the Golconda mines in India and made their way to Europe in the 17th century. Today, almost all blue diamonds come from the Argyle Mine in Western Australia and South Africa's Cullinan Mine. A few blue diamonds are found in other South African mines, Sierra Leone, the island of Borneo and in Brazil.

Blue diamonds are not only rare; they are challenging to cut into jewellery. Some blue diamonds have uneven colour across the crystal, called zoning. This makes it hard for a diamond cutter to create a cut diamond with consistent colour throughout the stone. The rough diamonds tend to be asymmetrical, making it more difficult to plan the cut and polish of the finished diamond. Both these issues lead to increased waste, further increasing the price per carat of the already very costly gemstones.

What Is A Blue Diamond?

All diamonds are made of carbon. The carbon is changed from its black from by immense heat and pressure over millions of years into the clear, brilliant crystals we use as jewellery. Blue diamonds take their colour from traces of the element boron which is forced into the crystalline structure of the diamond. Nitrogen atoms commonly found in clear diamonds can also be found in blue diamonds. The more nitrogen atoms that are present, the less intense will be the blue colour.

Many blue diamonds are not pure blue, they may be seen to have a greyish or greenish tinge to them. The greyish effect is due to hydrogen atoms being incorporated into the crystalline carbon structure. The green effect is due to exposure to natural radiation near to the Earth's surface.

Colour Treated Blue Diamonds

Not all blue diamonds are that colour when they are mined. Because they are so rare, many blue diamonds have been treated to make them appear blue. There are several colour treatments: irradiation, high pressure high temperature (HPHT), annealing, and surface coatings. All these processes can create a beautiful blue diamond; however, only irradiation creates a change that is regarded as being permanent. HPHT and annealing are both subject to change under the high heat used in jewellery repair, and surface coatings can be scratched or chipped. Check your diamond's grading report to confirm the treatment type and thus its permanence.

Figure 1. All the possible hues and intensities of a blue diamond. Courtesy: https://www.leibish.com/about-fancy-blue-diamonds-article-348

1st row: Light Blue, Fancy. Light Blue, Fancy Blue, Fancy Intense Blue, Fancy Vivid Blue, Fancy Deep Blue, Fancy Intense Violetish Blue,

2nd row: Fancy Light Grey-Blue, Fancy Grey-Blue, Fancy Dark Grey-Blue, Fancy Greyish Blue, Fancy Deep Greyish Blue, Fancy Light Greenish Blue, Fancy Intense Greenish Blue

3rd row: Fancy Vivid Greenish Blue, Fancy Green- Blue, Fancy Intense Green-Blue, Fancy Vivid Green- Blue, Fancy Deep Green- Blue, Fancy Greyish Greenish Blue.

How Are Blue Diamonds Graded?

Diamonds are assessed using the 4Cs of Colour, Clarity, Cut, and Carat Weight devised by the Gemological Institute of America. (GIA). For coloured diamonds, the colour and its quality are emphasised in grading assessments. The colour-grading scale is different from that used for classic, clear diamonds.

Blue Diamond Colour

Colour is assessed by skilled graders who decide the primary colour, in this case blue, and any secondary colour. The colour combination and balance between the primary and secondary colour are recorded in the form of a one- or two-word description. A pure blue diamond is marked as 'Blue'. If there is a secondary colour, which there usually is, then it is recorded as, for example, grey, if the secondary colour is strong, or greyish, if the colour is weak. See Figure one and its caption for the full range of colours applicable to blue diamonds.

Blue Diamond Intensity

The second aspect to diamond colour is how deep, or intense the colour is. The GIA grading system uses a series of nine descriptions, not all of which are used for every diamond colour. Pure blue diamonds are graded using only seven of the intensity grades: Faint Blue, Very Light Blue, Light Blue, Fancy Light Blue, Fancy Blue, Fancy Deep Blue, and Fancy Vivid Blue. If there is a secondary colour, then an additional grade Fancy Dark may also be used.

Diamond Colour Source

Given the rarity and cost of blue diamonds, it is no surprise that less expensive and easier to find alternatives are on the market. A jeweller should always disclose whether a diamond you are looking at is naturally coloured or treated to enhance or create the colour that you see. Not all jewellers do this, sometimes because the treated gems are almost impossible to distinguish from the real thing, other times for less creditable reasons. A colour grading report from a reputable grading lab such as the GIA will put your mind at rest about the nature of the diamond you are looking at. A report from the GIA will tell you if a diamond is natural or treated and, if treated, will show you how the colour was created. GIA does not report on synthetic, lab-grown diamonds.

Most of the blue diamonds that are sold for jewellery will have had their colour either enhanced or treated to create the beautiful gem that you see. If you know what you are buying and have not been misled, there is no problem with treatments or enhancements. A colour treated diamond is a beautiful, natural diamond at a relatively affordable price. Diamond collectors will want to buy natural diamonds and be willing to pay the very high prices for such stones. For jewellery, where the beauty of the piece is the most critical factor then choose a magnificent blue diamond that falls within your budget and enjoy the pleasure of ownership!