Fancy diamonds are diamonds which are richly colored, rather than clear or white. True fancy diamonds are rare, and most of the colored diamonds on the market are artificially colored through a variety of laboratory techniques. The value of a fancy diamond can range considerably, depending on the hue and intensity of the diamond, along with the cut and carat weight. The popularity of fancy diamonds for formal jewelry has waxed and waned, although some of the most famous diamonds in the world, such as the Hope Diamond, are fancy diamonds.

The unique colors of fancy diamonds are naturally caused by impurities in the stone. The high pressures which diamonds are subjected to while they are forming often admit impurities, which can detract from the price if they only faintly discolor the stone. If the impurities appear in large amounts, however, the diamond will be considered fancy, and can command a very high price, especially if the color is rare. Nitrogen and boron both cause colors to appear in fancy diamonds, as does irradiation, which causes the diamond to turn green. When artificially coloring diamonds in the lab, they are subjected to radiation, heat treatment, or high pressure.