Stars (known as Asterism) form on certain gemstones like star sapphire, star ruby, star garnet, star rose quartz, star diopside, star tourmaline, star spinel, star peridot, etc.
You can have 4 ray stars, 6 ray stars and 12 ray stars.
Star form on a cabochon gem. The cabochon shape of the gem focuses the light unto the surface of the gem. Near the surface of the gem, there are fine needles orientated in a certain way. When light hits these needles, light gets refracted and reflected forming the star effect.
Star gems show the star when they are under sunlight or any focused light source.
Star gems are great gifts as they are unique and they have many symbolic meanings due to the star effect.