If you're thinking of buying a beautiful piece of ruby jewelry, you need to look at the design of the piece and the stone itself. The three most important factors that rubies are judged on are hue, saturation and tone.
The hue, or color, of a ruby is the first thing most people notice about a stone. In fact, high-quality rubies are cut to maximize the quality of their color. A vivid, intense red is the most desirable color. Rubies from the Mogok Valley in Burma, for example, are known for their intense "pigeon blood" color and are very valuable. Variations in ruby color include red with yellowish to orange tones and red with blue to purple tones. Of course, a ruby is simply red corundum- any piece of corundum with a color other than red is called a sapphire.
The saturation, or depth, of color is also very important. It is accepted that nearly all rubies available commercially have been treated in some way to improve their saturation. A process called surface diffusion, in which a bland-looking ruby is heated to a high temperature in the presence of coloring agents, is one of these treatments. This applies a shallow layer of red coloring on the surface of the stone that gives a deep, saturated effect. Dyes and colored resins, which are intended to fill small cracks in the surface, can also serve the purpose of enhancing the color and saturation in a piece of ruby jewelry.The tone refers to the lightness or darkness of the color. Generally, a darker stone will have a more intense color, but the medium-toned rubies are usually the most prized. If you have a personal preference, or if the ruby is mounted with other types of gem that share a common tone, you may choose to go lighter or darker to suit the situation (Tom Webster).
