The value of a diamond is largely determined by its rarity. When it comes to white (or colourless) diamonds, that rarity is found in the absence of colour, however this is turned on its head with fancy colour diamonds because the rarest are those saturated in the deepest colours. Red is the most uncommon (and hence valuable) colour, followed by pink and then blue and even very slight colour differences can have a profound effect on the price of a stone. Where combinations of colours are present in the same diamond this can also increase its desirability. Providing a stone shows colour in the face-up position it can be categorised as a fancy colour stone. The accurate grading of fancy colour diamonds is a sophisticated mix of art and science, which can only be achieved by highly trained individuals, working in a hi tech lab.
(Fancy coloured stones come in a vast array of shapes and colours)
The GIA (Gemmological Institute of America), the pioneers of many grading techniques, use an approach that takes into account the fact that not all coloured diamonds have the same depth of colour. For example, yellow diamonds can be found in a wide range of saturations, whereas blues cannot. Gübelin Gem Lab, Lucerne, Switzerland, have adopted the much-coveted "six-eyes" principle by which every stone is assessed and examined by 3 separate scientists in order to guarantee the results are free from the bias of any one person. The strength of colour within a diamond can vary enormously, from vivid to faint, and some colour mixtures have been given their own names to boost their marketing. Examples of this include the brown diamonds produced by the Argyle mine, that are often referred to as "Champagne" or "Cognac" stones, many purplish-orangey-brownish stones are sold as "rose-coloured" and some purplish tints are marketed as "mauve" diamonds.
(A 'Cognac' diamond from the world-renowned Argyle mine)
Blue diamonds are among the rarest, and most expensive, of all coloured diamonds. Their colouration comes from the presence of tiny quantities of boron that was present in the crystalline structure of the stone when it was created deep within the earth's crust over 500 million years ago. Blue diamonds usually have a slight hint of gray, making their appearance slightly less intense than a blue sapphire, although the greater the quantity of boron in the stone, the deeper the blue. Blue diamonds are often colour enhanced with a variety of heat and other treatments in order to 'deepen' their blue hue. Even stones that have undergone this treatment can still fetch tens of thousands of dollars per carat and untreated deep and vivid blues have been known to reach hundreds of thousands of dollars at auction.
(Blue diamonds are among the scarcest of all)
Fancy green diamonds are interesting in that they are rarely green through the entirety of the stone, as most of the colouration is to be found at or near the surface. Due to this phenomenon, most cutters and polishers of green diamonds try to leave as much of the natural rough around the girdle as they can. Fancy green diamonds are usually quite light in tone and low in saturation, frequently influenced by a slight grayish or brownish cast and the hue is usually yellowish green. Green diamonds derive their colour from naturally occurring radiation in the earth (although this can be synthesised in a laboratory, in a process known as irradiation), which displaces the carbon atoms from their normal seats within the crystalline structure. It is very hard to tell a natural green diamond from an irradiated green but scientific methods do exist and the top gemmological laboratories are able to separate the two in most cases. Irradiated green stones can still fetch high prices and they also carry the benefit of being identical to their natural green counterparts to the naked human eye.
(The "Dresden Green," one of the largest fancy vivid green stones ever produced)
Brown is by far the most common fancy coloured diamond and it also holds the accolade of being the first to be used in jewellery manufacture. This is a practice which can be traced back in the archaeological records as far as antiquity, when Roman craftsmen were known to have set brown diamonds into rings as early as the second century AD. However over the many centuries that followed their popularity waned and as late as the 1980s they were largely relegated to purely industrial usage. This all changed with the discovery of the Argyle mine in Western Australia in the late 1970s. Since the start of large scale mining activities at the Argyle mine in the early 1980s, over 80% of the stones extracted have been brown diamonds. By means of clever marketing strategies and by employing cutters and polishers in India to reduce overheads, Rio Tinto Diamonds, the owners of the Argyle mine, have been able to develop a new and vibrant jewellery market for polished brown stones. Brown diamonds range from very light to very dark and buyers tend to favour the darker tones and those with a hint of other colours, including green, yellow, orange and especially reddish pink.
(Brown stones were ignored by jewellers for many years but they have experience a revival of late)
After brown, yellow is the second most common fancy coloured diamond. As with their brown counterparts, yellows have often been given catching names, such as "canary," for marketing and advertising purposes. Increasing celebrity endorsement from a variety of sources, including Heidi Klum and many Bollywood stars, has boosted the market for yellow diamonds, especially in Asia. Until very recently there was almost no market whatsoever for black diamonds, however this has started to change in recent years and they are now sometimes used to contrast white diamonds in pavé settings. They have also been given a boost by British F1 superstar Lewis Hamilton, who often sports black diamond earrings at red carpet events and in interviews. Fancy white diamonds do also exist with a milky white saturation and can be cleverly cut to give off stunning opalescent flashes of colour. Gray diamonds are coloured by elements of hydrogen in the chemical composition and again attempts have been made to market these stones as "chameleon" diamonds because their colour can appear to change slightly in different levels and angles of light.
(An example of a beautiful fancy vivid yellow diamond)
Although colour is the dominant determiner of price with fancy coloured diamonds (even a heavily included stone of the right face-up colour and depth of colour can still command a hefty price) clarity does have a secondary role to play. Very heavily included stones can be more brittle and susceptible to damage, which can in itself have a significant effect on their market price. Fancy coloured stones can experience colour 'graining,' which is consider by gemmologists to be a form of inclusion.
(GIA clarity scale: from 'Internally Flawless' to 'Included 3')
The cut and carat size of a diamond can actually influence its colour. The larger a stone, or the deeper its pavilion, the deeper light can pass into it, which can lead to a deeper, more intense saturation. Different cuts can also have a profound effect on the colour of the resulting stone. Mixed cuts, like radiant, can work well to deepen the colour in a yellow diamond. This perceived improvement in colour can add significantly to the price per carat achieved when the stone is polished. The radiant style also affords a higher yield per carat of rough versus a round brilliant.
(some of the most well known cuts, or shapes)
In simple terms the larger the carat the higher the price, all else being equal! The price of a stone of a given cut, colour and clarity will increase exponentially along a bell-curve with increases in carat size. Consequently a stone twice the size of another, and it's exact equal in all other respects, could be many times more valuable. There are key carat size points over which prices can increase dramatically. For example, a 0.98 carat stone may look identical to a 1.00 carat diamond but there could be a significant difference in price. The same can also be true of half and even quarter carat points with smaller stones. This presents an opportunity for the savvy buyer!