For many engagement ring buyers, the preferred diamond colour is white – indeed, the whiter, the better (and the pricier!). A pure white diamond, often known as blue-white and graded colour ‘D’ is exceptionally hard to come by and these stones demand premium prices. As you move down the diamond colour scale (and down the alphabet towards a grading of ‘Z’) the diamond becomes less white and tends towards pale yellow colours. These ‘tinted’ diamonds are considered to be of an inferior colour and consequently fetch lower prices.

At a stage however, the yellow colour in a diamond is so vivid and so intense that it crosses out of the traditional diamond colour chart and is considered a ‘fancy’ coloured diamond. Fancy coloured diamonds are exceptionally rare and exceptionally valuable, commanding prices of 10’s and 100’s of thousands of dollars. But what causes coloured diamonds? Why are some white and some not-so-white and some blue or pink or brown or red?

Quite simply, the colour in diamonds is caused by chemical impurities or deformation of the diamond crystal structure. Ironic, when you consider that on the A-Z diamond colour scale, less pure means less value!

Let’s start with white diamonds. Diamond is comprised almost entirely of the element Carbon and a pure blue-white diamond (graded colour ‘D’) will contain no chemical impurities – it is a pure form of Carbon. However, the area of the earth where diamonds grow is essentially a chemical hot-pot, with many other elements waiting to be snapped up by minerals as they grow.

One of the more common elements is Nitrogen and, when a diamond has a significant amount of Nitrogen in its structure, the diamond takes on a yellow colour. The more nitrogen in the structure, the more yellow the diamond. The presence of nitrogen can also impart an orange colour, but orange diamonds are exceptionally rare.

Violet coloured diamonds are also very rare and are believed to be caused by the presence of hydrogen in the diamond structure. Blue diamonds are amongst the most prized colours and are found in only a limited number of areas worldwide. Blue diamonds are caused when the element Boron is incorporated during the growth of a diamond. As is the case with Nitrogen and yellow diamonds, the more Boron there is in a diamond, the darker the blue colour will be.

Black diamonds are the result of a massive number of imperfections and inclusions (often graphite inclusions) within a diamond. There are so many inclusions that the diamond is almost opaque and allows almost no light to pass through, resulting in a black appearance. Black diamonds can almost be considered as industrial diamonds and are thus traditionally priced lower than white diamonds. The degree of imperfections makes black diamonds very hard to cut and you will often only find black diamonds cut into round brilliant shapes.

Brown diamonds and pink diamonds are thought to be the result of the same process – crystal deformation. Diamonds, like other crystals, have a regular arrangement of atoms and bonds, but under pressure, these atoms and bonds can shift out of position and it is this shift that results in a colour change to either brown or pink, possibly depending on the degree of shift. This process is also believed to account for red and purple diamonds that are occasionally discovered.

Green diamonds are exceptionally rare and are the result of a diamond having been subjected to radiation, normally deep within the earth’s crust or mantle, millions of years ago. Don’t worry though, if you have a green diamond, it’s not radioactive!

An important fact to bear in mind when buying coloured diamonds is to ask the retailer or diamond dealer whether the diamond colour is natural or enhanced. Many laboratory techniques can be used impart, improve or alter the natural colour of diamonds and such information must be given to the diamond purchaser.