Star Rubies are an uncommon variety of the precious stone Ruby. These glorious gems exhibit a sharp six-rayed star that appears to glide marvelously across the surface of the gem. The glistening star is best visible when the star ruby is seen under sunlight or spotlight. Researchers suggest that the reason behind this star effect is an optical phenomenon that is called as "Asterism". It is triggered by the dense presence of minute fibers of rutile or silk.