Yellow diamonds are diamonds which have taken on a yellow tone, either through natural interruptions in their formation process or chemical treatment. Diamonds that are naturally this color are extremely rare, and the color is usually caused by nitrogen impurities that appear while the diamond is being formed. More commonly, yellow diamonds are synthesized in the lab; lab-made diamonds are chemically indistinct from natural diamonds, and are widely used in ornamental jewelry and some manufacturing processes.
Usually, clear diamonds are in high demand. A flawless clear diamond which is cut in a flattering way can fetch a great deal of money once set in a piece of jewelry, while a discolored diamond is not considered valuable. If, however, the color is strong and intense, it becomes what is known as a fancy diamond: a naturally colored diamond. Fancy diamonds such as yellow diamonds are handled and graded differently than clear diamonds, and some of the most famous diamonds in the world, such as the Hope Diamond, are fancy diamonds.
When yellow diamonds are being graded, independent assessors look at the stones to determine the basic hue and its intensity. Fancy diamonds come in red, purple, pink, brown, yellow, and sometimes green; the first thing to determine is what color the diamond is. Next, the intensity of the diamond is gaged on a scale that runs from fancy faint, meaning that the color is barely perceptible, to fancy vivid, meaning that the color is rich and intense. Next, graders check the stones for inclusions, although minor inclusions are not as noticeable as they are in clear ones. Finally, the stone must be well cut to enhance the stunning color.
A number of names are used to refer to yellow diamonds commercially including canaries, a reference to their color, and Cape series, a reference to the African mines where these diamonds are most commonly found. Since the diamonds rarely appear naturally or often have weak color, some companies treat their diamonds with irradiation and heat to lend or enhance color. Heat treatment to create colored diamonds appears to be permanent, and the color should not fade or change over time unless the diamond is exposed to heavy radiation again.
Finding true natural yellow diamonds can be very difficult, due to chemical synthesis and lab diamonds. Not all jewelers can be certain about the provenance of their stock, especially in small jewelry pieces, so prepare to pay a hefty sum for a true certified yellow diamond. Because most of these diamonds come from Africa, you may also want to ask questions about the jeweler's source, to guard against purchasing a blood or conflict diamond.