overall quality. Sapphires that are completely untreated are worth far more than those that have been treated. Geographical origin also has a major impact on price. For most gems of one carat or more, an independent report from a respected laboratory such as GIA, Lotus Gemology, or SSEF, is often required by buyers before they will make a purchase. Colors Sapphires in colors other than blue are called "fancy" or "parti-colored" sapphires. Fancy sapphires are often found in yellow, orange, green, brown, purple and violet hues. Blue sapphire Gemstone color can be described in terms of hue, saturation, and tone. Hue is commonly understood as the "color" of the gemstone. Saturation refers to the vividness or brightness of the hue, and tone is the lightness to darkness of the hue. Blue sapphire exists in various mixtures of its primary (blue) and secondary hues, various tonal levels (shades) and at various levels of saturation (vividness). Blue sapphires are evaluated based upon the purity of their blue hue. Violet and green are the most common secondary hues found in blue sapphires. The highest prices are paid for gems that are pure blue and of vivid saturation. Gems that are of lower saturation, or are too dark or too light in tone are of less value. However, color preferences are a personal taste, like a flavor of ice cream. The Logan sapphire in the National Museum of Natural History, in Washington, D.C., is one of the largest faceted gem-quality blue sapphires in existence. The 422.66-ct Siren of Serendip in the Houston Museum of Natural Science is another stunning example of a Sri Lankan sapphire on public display. Parti sapphires Particolored sapphires (or bi-color sapphires) are those stones that exhibit two or more colors within a single stone. The desirability of particolored or bi-color sapphires is usually judged based on the zoning or location of their colors, the colors’ saturation, and the contrast of their colors. Australia is the largest source of particolored sapphires; they are not commonly used in mainstream jewelry and remain relatively unknown. Particolored sapphires cannot be created synthetically and only occur naturally. The vast majority of particolored sapphires occur naturally, but it is possible to replicate the appearance of a particolored sapphire in a synthetic sapphire. Colorless sapphires have historically been used as diamond substitutes in jewelry. Pink sapphires Pink sapphires occur in shades from light to dark pink, and