Blue star sapphires are a rare variety of sapphire that exhibits a phenomenon known as asterism. This occurs when the sapphire is cut into a cabochon (a smooth, rounded shape with a flat bottom), and reflections from tiny needle-like inclusions of the mineral rutile are aligned in such a way that they form a star-like pattern on the surface of the stone. The star is best viewed under direct light, such as sunlight or a focused lamp. The value of a blue star sapphire can be influenced by the intensity and attractiveness of the body color, the visibility and sharpness of the star, and the overall clarity of the gemstone.
