Jewelers describe the color of a gemstone in terms of three characteristics: hue, saturation, and tone. A gemstone's basic color 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 color can be detected. Saturation is a measure of the intensity or purity of a gem's hue and is determined by the degree to which gray or brown hues mute its defining color. 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.