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,
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 423-carat (84.6 g) 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.
Dark blue sapphire, probably of Australian origin, showing the brilliant surface luster typical of faceted corundum gemstones.
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