Sapphire is the gem variety of corundum that is not red ruby. Blue may be its best-known expression, but natural sapphire also occurs in yellow, pink, green, purple, orange and colourless material, as well as stones that shift appearance under different light.
Colour can be described through hue, tone and saturation. Hue is the colour family, tone is how light or dark it appears, and saturation is its intensity. Two sapphires labelled blue may therefore have very different personalities.
Cut plays an important role because sapphire rough often contains zoning. A sensitive orientation can blend colour through internal reflections, while an unsuitable orientation may leave one side inky and another pale.
View sapphire in more than one lighting environment. Daylight, warm interior light and directional spot lighting can each emphasise different qualities. The most satisfying choice is not necessarily the most saturated stone; it is the one whose colour remains engaging in the conditions where it will be worn.