Black star sapphires, also known as "asteriated sapphires," display a unique optical phenomenon called asterism, which creates a star-like pattern on the surface of the stone when viewed under a direct light source. This effect is caused by the presence of needle-like inclusions of the mineral rutile within the sapphire crystal lattice. The asterism typically manifests as a six-rayed star that moves across the surface of the gem as it is rotated, adding to its allure and mystique.
