Star Sapphires display a 6 rayed star - By light reflecting off the silky crystallizations of rutile inside called asterism.
Star Sapphires are Corundum from translucent to opaque and comes in a range of colors but the finer Blues, Reds, Green yellow orange and Padparaschas are extremely rare.
Some color-change Corundum also displays asterism, and are called Color-change Star Sapphires which are rare and sought-after by collectors and connossiers alike.