A 13.22-carat blue diamond, the largest in its category, fetched a total of US$23.79 million (S$29.7 4 million) at a Christie's auction in Geneva on Wednesday.
At another auction by Sotheby's in Geneva on Tuesday, a 100-carat flawless yellow diamond was sold for US$16.3 million.
Such marquee sales underline the growing popularity of coloured diamonds. Once considered a curiosity, they are rarer than white diamonds and now attract higher prices per carat than even the most flawless, translucent stones.
The colour in these diamonds is produced by certain chemical impurities or structural defects within the crystal lattice work of the diamond.
Coloured diamonds are evaluated less for brilliance and more for color intensity. Shades that are deep and distinct are rated higher than weak or pale shades.
The colour is generally described in terms of hue, tone and saturation. Hue refers to the diamond's characteristic colour, tone refers to the colour's relative lightness or darkness, and saturation refers to the colour's depth or strength.
Types of coloured 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 jewellery. They gave the diamonds names like "cognac" and "champagne." The marketing worked, and brown diamonds are found in many medium-priced jewellery designs today.
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.
Yellow is the 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 these diamonds. But designers started using them in jewellery, especially contrasted with tiny colourless diamonds in pavé settings, and they began to gain in popularity.
Blue diamonds are extremely rare. They generally have a slight hint of grey and 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.
Fancy green diamonds are typically light in tone and low in saturation. Their colour often appears muted, with a greyish 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.
Most grey diamonds contain a high level of hydrogen as an impurity element, which probably causes their colour.
How are fancy colour diamonds rated?
According to the Gemological Institute of America (GIA), a scale of nine grades is used to grade fancy colour diamonds. The higher the intensity, the more valuable is the diamond:
2. Very Light
4. Fancy Light
6. Fancy Dark
7. Fancy Intense
8. Fancy Deep
9. Fancy Vivid
Source: Gemological Institute of America