When such material is cut into a cabochon the sheen is concentrated along the top of the stone into three white lines crossing at 120° angles, creating a six-rayed star. The rutile needles orient themselves according to the hexagonal symmetry of the corundum, and reflections from these needles provide a chatoyancy. Star corundum gemstones show asterism, or a "star effect." This phenomenon is created by the inclusion of rutile needles within the host corundum crystal. Among sapphires the green gems are more iron-rich, have higher indices.ĬORUNDUM: Star ruby, Sri Lanka (2.75), Burma (2.6, 0.62), India (8.4). Synthetics: See Identifying Inclusions and Synthetic Gemstones and Their Identification.Ĭharacteristics of Corundum from Various Localities
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