Asterism is a type of optical phenomenon caused light reflecting off impurities inside of gemstones, such as fibrous materials arranged in specific directions, mineral, gas or liquid inclusions. When the gemstones are cut en cabochon in specific directions, light reflected off the surfaces of these gemstones will spread out into a star-like pattern, hence give rise to the name asterism (star-like). There are usually 4 or 6 rays of light off the centers of these asterisms, which are related to how a mineral crystallizes. Star rubies and sapphires are valuable gemstones with asterisms; they belong to the hexagonal crystal system. To produce the desired star light effect, one must select gemstones with specific types of inclusions and perform cuts along the perpendicular axis (C axis) to achieve good quality asterism.