corundum, sapphire, star sapphire, inclusions.5, star quartz, gems

Weibel, Wessicken, Woensdregt, and Wüthrich examine the optical mechanism responsible for chatoyancy and asterism, with particular attention to black star sapphire from southeastern Thailand and milky-white star quartz from Ratnapura, Sri Lanka. They explain that each ray of a star results from light scattering by a set of parallel fibrous structures within the crystal, with refraction at the curved cabochon surface also contributing to the observed effect. Contrary to the usual explanation involving rutile silk, electron microscopy revealed no inclusions in the Thai black star sapphire; its fracture surfaces instead show stepped structures with trigonal dislocations, and the authors suggest regular lattice dislocations or twinning as possible causes of the asterism. In the Ratnapura star quartz, electron diffraction identifies the scattering needles as sillimanite, averaging about 0.02 μm thick, rather than rutile; their imperfect orientation accounts for the diffuse and incomplete star.