gems, spinel, Sri Lanka

An unusually coloured natural spinel examined after the 1994 Tucson gem show provides evidence that nickel can contribute to the colour of natural spinel. The 4.88-carat cushion-cut stone, reportedly from Myanmar, displayed a slightly greenish blue or “teal” colour rather than the saturated royal blue more commonly associated with natural cobalt-bearing spinel. Standard gemmological testing gave a refractive index of 1.714, anomalous double refraction, and a specific gravity of 3.60. The specimen was inert to long-wave and short-wave ultraviolet radiation, fluoresced red under visible light, and showed a moderate red reaction through a colour filter. A handheld spectroscope revealed broad absorption bands near 460, 550, and 585 nm, together with total absorption above 610 nm, consistent with natural cobalt-blue spinel. Microscopic examination found the stone essentially clean apart from elongated yellow stains in surface-reaching fractures near a corner of the table; these could be mistaken for flux inclusions. Energy-dispersive X-ray fluorescence analysis supported natural origin through substantial zinc and gallium contents typical of natural spinel. In addition to magnesium and aluminium, the analysis detected abundant iron, trace manganese, and an unusually high nickel content compared with other natural blue spinels in the laboratory’s database. Cobalt was not detected, as its usual concentration in natural cobalt-blue spinel lies below the instrument’s detection limit. Ultraviolet–visible absorption spectroscopy identified a transmission window near 500 nm, compared with approximately 490 nm in typical royal-blue cobalt-bearing spinel. This shift towards greenish blue was attributed to a broad absorption underlying the principal cobalt feature. Absorption at 480 nm was also approximately as intense as that at 460 nm, whereas the former is usually weaker in other natural cobalt-blue spinels. Comparison with a Russian flux-grown synthetic spinel doped with divalent nickel suggested that nickel caused the additional absorption, consistent with the chemical analysis. The authors regarded the specimen as the first reported example of nickel contributing to the colour of natural spinel.