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Star or Cat’s Eye Ruby and Sapphire

A star ruby and sapphire is a type of corundum that exhibits a star-like phenomenon known as asterism; red stones are known as "star rubies". Star ruby and sapphires contain intersecting needle-like inclusions following the underlying crystal structure that causes the appearance of a six-rayed "star"-shaped pattern when viewed with a single overhead light source. The inclusion is often the mineral rutile, a mineral composed primarily of titanium dioxide. The stones are cut en cabochon, typically with the center of the star near the top of the dome. The inclusions can alternatively produce a “cat's eye”effect, if the “face-up” direction of the cabochon's dome is oriented perpendicular to the crystal's c-axis rather than parallel to it. If the dome is oriented in between these two directions, an “off-center” star will be visible, offset away from the high point of the dome.