Fancy Colours
Artificial Colour Enhancements
Depending on initial properties, the colour of diamonds can be changed from yellow or brown tints into either white colourless, or a fancy colour of many possible shades. Such enhancements are usually done by irradiation, high pressure - high temperature processing, or simple application of coloured coatings.
Irradiation
Irradiation is one of the most popular methods. Bright blue, green, pink, yellow or purple colour diamonds can be produced out of a regular yellow tinted stone. This process requires several steps. At first, the diamond is exposed to the source of radiation (it may be all kinds of elementary atomic and subatomic particles) and reaches a blue or blue-green colour. The resulting diamond then is heated to specific high temperatures, usually several times, until it reaches the desired colour. This treatment is permanent and safe. The stone can be re-polished safely, as the colour goes deep into the surface. Even rough stones can be irradiated prior to cutting.
Irradiation works by slightly changing the internal structure of the crystal, altering how the crystal reflects and absorbs different wavelengths of light. A completely white diamond reflects all wavelengths, while a coloured diamond absorbs some of them. A spectroscope can be used to determine which wavelengths exactly are absorbed, and the pattern seen with a spectroscope can serve as a fingerprint for the type of diamond in question. A diamond of every colour and origin has its own fingerprint. This is because the precise frequencies absorbed by the stone are dependent on its structure. Naturally coloured diamonds of pink, yellow or blue colours have not been irradiated and their crystal structures are not identical to those that have been. This means that with proper equipment it is fairly easy to distinguish natural and artificial colour sources of these diamonds. The green diamonds, however, both mined in their green colour and enhanced, are produced only by irradiation, and so their light absorption patterns will be the same in both cases. All colour enhanced diamonds still remain real diamonds, and are not considered artificial in and of themselves. The prices of colour enhanced stones will definitely be much lower than the prices of natural intense coloured diamonds, closer to those of equivalent colourless diamonds.
Some would prefer a naturally coloured gemstone such as a sapphire to an enhanced coloured diamond because the sapphire is seen as a more natural choice with no modifications to its colour. This is often not true since it is a common practice to heat sapphires to saturate their colours. That aside, any diamond is still 140 times harder than a sapphire crystal and therefore more durable. Even when the two have the same general hue, diamonds and sapphires have a different look. Some people can see a big difference in the sparkle of a vivid yellow diamond when compared to a yellow sapphire, assuming both have optimal cuts.
High Pressure - High Temperature
Also known as HPHT, this treatment is used to make off-color diamonds (faint yellow and brown) significantly whiter. Often colours of at least F to E can be produced. It works by heating the stone in a highly pressurized chamber, similar to the way man made diamonds are created. Only certain types of diamonds can be treated this way. While this treatment is permanent, it is easily detectable in a lab, because the diamonds retains small spots where the color has not been fully eliminated. These spots of course are very small and invisible to the naked eye, they may even be invisible with a standard 10x loupe. Diamonds treated this way will also have extra absorption lines that are uncommon for untreated white diamonds. Like with irradiation, the use of HPHT treatment has to be disclosed at the point of sale and will generally lower the price of the diamond. In normal wear, however, these diamonds will look the same way as untreated ones, and be just as durable.
Coated Diamonds
Coated diamonds are simply what they claim to be: coated in a specific dye that will make them appear whiter or fancy coloured. This treatment is very undesirable, since the coating can get scratched and the diamond will have an uneven colour. It will also get damaged if the diamond is re-cut or re-polished, as the coating is limited to only a thin layer on the surface. Diamonds treated this way are much cheaper than any other, since their colour does not last long and is very vulnerable to damage.
