In diamonds, rarity equals value. With diamonds in the normal range, value is based on the absence of colour, because colourless diamonds are the rarest.
NATURAL FANCY COLOUR DIAMONDS
With fancy colour diamonds—the ones outside the normal colour range—the rarest and most valuable colours are saturated pinks, blues, and greens. In all cases, even very slight colour differences can have a big impact on value.
Compared to fancy yellows and browns, diamonds with a noticeable hint of any other hue are considerably more rare. Even in light tones and weak saturation, as long as they show colour in the face-up position, they qualify as fancy colours. Red, green, and blue diamonds with medium to dark tones and moderate saturations are extremely rare.
Grading fancy colour diamonds is complex and specialized, and it takes highly trained laboratory graders to complete the process accurately.
The GIA system for colour-grading fancy colour diamonds is designed to accommodate the fact that not all coloured diamonds have the same depth of colour. For example, yellow diamonds occur in a wide range of saturations, while blue diamonds do not.
Natural Red and Pink Diamonds
Diamonds with red or reddish colours are extremely rare and highly valued. Pure pinks are more popular than diamonds that are purplish, orangy, brownish, or grayish. Trade professionals market some very attractive stones in this category as “rose-coloured,” and some stones with purplish tints as “mauve” diamonds.
Natural Blue Diamonds
Blue diamonds are extremely rare. They generally have a slight hint of gray, so they’re rarely as highly saturated as blue sapphires. Their colour is caused by the presence of boron impurities—the more boron, the deeper the blue.
Natural Green Diamonds
Fancy green diamonds are typically light in tone and low in saturation. Their colour often appears muted, with a grayish or brownish cast. The hue is generally in the yellowish green category. In most green diamonds, the hue is confined to the surface, and rarely extends through the entire stone. That’s why cutters try to leave as much of the natural rough around the girdle as possible.
Green diamonds get their colour when radiation displaces carbon atoms from their normal positions in the crystal structure. This can happen naturally when diamond deposits lie near radioactive rocks, or artificially as a result of treatment by irradiation.
Naturally coloured green diamonds are extremely rare. Because of their rarity and the very real possibility of treatment, green diamonds are always regarded with suspicion and examined carefully in gemological laboratories. Even so, advanced gemological testing can’t always determine colour origin in green diamonds.
Natural Brown Diamonds
Brown is the most common fancy diamond colour and also the earliest to be used in jewellery. Second-century Romans set brown diamonds in rings. In modern times, however, they took some time to become popular.
Brown diamonds were typically considered good only for industrial use until the 1980s, when abundant quantities of them began to appear in the production of the Argyle mines. The Australians fashioned them and set them in jewelry. They gave them names like “cognac” and “champagne.” The marketing worked, and brown diamonds are found in many medium-priced jewelry designs today.
Natural Brown Diamonds
Brown diamonds range in tone from very light to very dark. Consumers generally prefer brown diamonds in medium to dark tones with a warm, golden to reddish appearance. They generally show a hint of greenish, yellowish, orangy, or reddish modifying colors.
Natural Yellow Diamonds
Yellow is diamond’s second most common fancy colour. Yellow diamonds are sometimes marketed as “canary.” While this isn’t a proper grading term, it’s commonly used in the trade to describe fancy yellow diamonds.
Until the late 1990s, there was not much demand for black diamonds. But designers started using them in jewelry, especially contrasted with tiny colourless diamonds in pavé settings, and they began to gain in popularity.
Source: Gemological Institute of America