Corundum Star Gemstones must be cut as cabochons in order to exhibit asterism. The cause of the asterism is exsolved rutile or hematite ‘silk’, multitudes of tiny oriented needles or plates. These inclusions form by exsolution – the unmixing of a solid solution. At high temperatures, the corundum may absorb more impurities, but as the crystal cools, the impurities can sometimes be forced out of solution. Since they are still in a solid, the impurity atoms only crystallize where there is space and this is determined by the host structure. In corundum, exsolved rutile and hematite crystallize in three directions in the basal plane, parallel to the faces of the hexagonal prism. When a stone contains enough of such inclusions, it can display a sixrayed star effect if it is cut as a cabochon, with the base of the cab parallel to the basal pinacoid (parallel to the layers of silk). The hematite crystallizes parallel to the first-order hexagonal prism while the rutile follows the second-order hexagonal prism (30 off the first-order prism). If a stone contains enough of both hematite and rutile silk, it can display a twelverayed star. This is fairly common in black star sapphires, but is rare in other colours. To observe the star effect the stone should be examined in direct sunlight, or under a single, intense light, such as a fibre-optic light, pen light or spot light.