corundum, inclusions.5, Sri Lanka.12, gems

Carbonin, Sbrignadello, and Ajò demonstrate the value of combining independent analytical techniques to identify solid inclusions in corundum through a study of a commercially obtained blue sapphire containing thorite. The stone was provisionally assigned to a Sri Lanka type on the basis of an experienced supplier’s assessment, but reliable information establishing its geographical provenance was unavailable. Transmitted-light microscopy revealed at least two visually distinct inclusion types. A red, well-formed octahedral crystal associated with fingerprint inclusions was considered to be chromium-bearing spinel, a feature previously described in Sri Lankan sapphire. Photoluminescence measurements were undertaken with helium–neon laser excitation and subsequently with a more intense argon laser because the initial chromium-related emission from the host sapphire was weak and noisy. The principal identification problem concerned a second inclusion, approximately 45 µm long, whose habit, surrounding tension haloes, and fissures initially suggested metamict zircon. Because this grain lay close to the sapphire’s surface, the host was polished until the inclusion was exposed for electron microprobe examination. Energy-dispersive analysis unexpectedly detected no characteristic zirconium radiation. Instead, silicon and several diagnostic thorium emissions dominated the spectrum, supporting identification as thorite, ThSiO₄, whose crystal habit closely resembles that of zircon. The grain’s extremely small size prompted confirmation by an independent method. Low-background gamma spectrometry using an intrinsic germanium detector established the presence of natural thorium-232 through characteristic emissions from its radioactive daughter products, lead-212 and thallium-208, at 238.6 and 583.1 keV, respectively. The agreement between elemental microanalysis and nuclear spectrometry strengthened the thorite identification beyond what could be inferred from optical appearance alone. The authors emphasize that their complementary methods probe different physical processes, involving electronic and nuclear energy levels, and therefore provide independent support when a microscopic identification is uncertain. The study cautions against assigning zircon solely from crystal morphology and associated stress features, since another radioactive silicate can present a similar appearance. It also suggests that thorite should be considered among the inclusion species potentially relevant to sapphire provenance investigations, while the uncertain source of the specimen prevents its use as firm evidence of a Sri Lankan occurrence.