Though diamonds come in a wide range of colours, colourless diamonds have traditionally been considered the most valuable. Most diamonds are graded on a scale using the letters of the alphabet, from D (colourless), the best grade, through Z (a light yellow). It is difficult for the untrained eye to notice such variations in colour unless stones are being compared side by side. Truly colourless diamonds (D) are admired for their beauty and rarity and are therefore more valuable than lower graded diamonds which are more widely available. The choice of preferred colour is purely down to personal taste. The comparatively rare coloured diamonds are known as fancy colours and are also quite valuable. They range in hue from the more common yellow (also graded Z+ on the alphabetic scale) to pink, blue, green, red, and even black and white. Diamonds that are graded D, E, and F tend to be the most expensive because of their rarity. However, any diamond you buy should have a good balance of cut, color, and clarity, so as to impart the most brilliance and dazzle possible.
Jewellers describe the colour of a gemstone in terms of three characteristics: hue, saturation, and tone. A gemstone's basic colour is its hue, and those with purer hues (for emeralds, green; for sapphires, blue; and for rubies, red) are generally considered more valuable. Often, however, a hint of another colour can be detected. Sapphires will range from purplish blue to greenish blue; emeralds, yellowish green to bluish green; and rubies, orangey red to purplish red. Saturation is a measure of the intensity or purity of a gem's hue and is determined by the degree to which grey or brown hues mute its defining colour. Value tends to increase with saturation, so a fully saturated purplish blue sapphire may well be more expensive than a muted pure blue one. The tone of a gemstone, a measurement of its lightness or darkness, is usually given as light, medium-light, medium, medium-dark, or dark.