corundum, inclusions.5, gems

Verneuil-grown star corundums—both ruby and sapphire—display variable color zoning and contain three sets of acicular inclusions intersecting at 120°, measuring 20–30 μm in length and 0.1–0.4 μm in width. These inclusions, oriented parallel to the {110} planes and showing polysynthetic twinning on those planes, have cross-sections that approximate the wavelength of visible light, producing sharp six-rayed stars in cabochon-cut stones due to optical diffraction. High-resolution TEM and electron diffraction reveal the inclusions to be nearly isostructural with the corundum host, but distorted into a monoclinic lattice (a = 5.00 Å, b = 4.63 Å, c = 13.08 Å, γ = 118°). Nanochemical analysis indicates a TiO₂ stoichiometry, suggesting Ti⁴⁺ cations occupy half the octahedral sites in a corundum-like structure, with associated vacancies. Near the apical tips of the TiO₂ needles, local composition suggests the presence of Ti²⁺ ions, attributed to redox disproportionation occurring during post-growth thermal annealing.