Star (asteria) and cats eye stones.

A beautiful feature of some gemstones are the chatoyant properties that some posses. When light is on the stone a cross will appear (in the case of star/asteria stones) and a single line (for cats eye stones). The cross or line will move around the stone dependant upon the angle you are looking at it. These features can be found on cabochon cut gemstones, such as Sapphire, Ruby, Peridot and Chrysoberyl.

What causes this display?
The beautiful display is caused by several factors in the stones physical make up. One cause is small tubular cavities within the stone that will refract the light giving the effect. The angle of these cavities dictates how many points the star will have, whether it will be a cats eye, four point or six point star. Here are some of the types of star and cats eye stones.