The color azure ultimately takes its name from the intense blue mineral lapis lazuli. Azure also describes the color of the mineral azurite, both in its natural form and as a pigment in various paint formulations. In order to preserve its deep color, azurite was ground coarsely. Fine-ground azurite produces a lighter, washed-out color. Traditionally, the pigment was considered unstable in oil paints and was sometimes isolated from other colors and not mixed.

Source: Wikipedia