corundum, ruby, sapphire, inclusions.5, phenomena, star, gems

A.S. Tait investigates the cause of asterism in corundum, testing competing explanations involving twinning lamellae, microscopic cavities, and regularly arranged solid inclusions. The study draws on extensive rough and cut material from Montana, North Carolina, Ceylon, Siam, Burma, and Queensland, with basal thin sections permitting examination of inclusion arrangements in transmitted and reflected light without the distortion introduced by curved gemstone surfaces. Microscopic observations show that star-producing material contains rodlike or needlelike inclusions arranged in the basal plane, parallel to the hexagonal prism faces and intersecting in three directions at 60°. When these inclusions are sufficiently abundant and evenly distributed, a cabochon cut with its axis parallel to the crystal’s optic axis displays a six-rayed star. Stars formed in water droplets placed on thin sections provide a direct demonstration linking the optical effect to the included material. Tait also identifies a second set of basal inclusions oriented perpendicular to the first-order prism faces. Although generally too sparse to produce visible rays, these offer an explanation for twelve-rayed stars when sufficiently concentrated alongside the principal set. Determining the inclusions’ mineral identity proves more difficult. At magnifications up to 900×, they appear reddish brown in transmitted light and show apparently square cross-sections, suggesting rutile, but their fineness prevents reliable refractive-index measurement. Microchemical analysis and X-ray powder examination fail because the quantity of included material is too small. Spectrographic analysis therefore compares carefully prepared samples ranging from apparently inclusion-free corundum to heavily included star-producing material. The latter contains titanium at approximately 0.001% and iron at approximately 0.009%, with traces of chromium; the apparently inclusion-free material shows aluminium with traces of chromium and iron. Combining the titanium detection with the observed colour and crystal habit, Tait identifies the needles as rutile, excluding other titanium dioxide minerals on morphological grounds. The article concludes that symmetrically oriented rutile needles account for the asterism of the material examined. Photomicrographs further relate optical appearance to inclusion distribution: predominantly one-directional concentrations produce a cat’s-eye effect, localized arrangements restrict where a star can be obtained, and uniformly distributed needles permit star formation across suitably oriented portions of the crystal.