An asterism is an optical phenomenon displayed by some rubies, sapphires, and other gems (e.g. garnet, diopside, spinel) of an enhanced reflective area in the shape of a "star" on the surface of a cabochon cut from the stone. Star sapphires and rubies display the property from titanium dioxide impurities (rutile) present in them.[2] The star-effect or "asterism" is caused by the difference in refractive index between the host material and that of the dense inclusions of tiny fibers of rutile (also known as "silk"), the rutile having a greater R.I. causes the relative bright relief of a star in a host material such as corrundum which has a refractive index between 1.760 - 1.778 much lower than that of rutile. The stars are caused by the light reflecting from needle-like inclusions of rutile aligned perpendicular to the rays of the star.