gems, spinel, color-change spinel, Sri Lanka, cobalt spinel

A laboratory examination documents an unusual colour change in a 2.24-carat natural cobalt-coloured spinel mounted in a man’s ring. The stone appeared dark, richly saturated, slightly violetish blue under fluorescent illumination and dark purple under incandescent light. Its contemporary setting, combining white and yellow metals, enhanced the visual effect. Testing at the West Coast laboratory established properties consistent with natural cobalt-blue spinels examined previously, including examples reportedly originating in Sri Lanka, although the note does not establish a geographic source for this particular specimen. A Duplex II refractometer yielded a single refractive-index reading of 1.720. Polariscope examination showed a singly refractive response accompanied by very slight anomalous double refraction. The stone displayed weak red transmission and appeared moderately red through a Chelsea colour filter. Examination with a handheld spectroscope revealed the diagnostic absorption band of natural cobalt spinel at approximately 460 nm, together with additional bands near 480, 552, 559, 575, and 622 nm. Broad general absorption extended from approximately 660 nm towards longer wavelengths. These optical observations supported comparison with the laboratory’s earlier experience of natural cobalt-blue spinel despite the specimen’s unusual change between blue and purple. Microscopic examination beneath one end of the crown revealed a cluster of moderately sized inclusions comprising a well-formed octahedral crystal and several whitish crystals of irregular shape. Rain-like strings of pinpoint inclusions radiated from this group and extended through the stone, accompanied by numerous small, fine needles. The included crystals and needles are described by their appearance and morphology without assignment to specific mineral species. The report combines observations under different light sources with refractometry, polariscope testing, colour-filter examination, absorption spectroscopy, and microscopy to characterize the mounted gemstone. Its significance lies in documenting a distinctive blue-to-purple colour change in a stone whose other measured properties and internal features were consistent with natural cobalt-blue spinel.