gems, spinel, Tanzania, Morogoro, inclusions, spinel inclusions, högbomite inclusions

A 2.28 ct purplish pink faceted pear-shaped stone submitted to the West Coast laboratory was identified as natural spinel and reported by its owner to originate from Tanzania. Its refractive index was 1.714, and it was singly refractive with very weak anomalous double refraction. Hydrostatic specific gravity was approximately 3.60. The stone displayed strong red fluorescence under long-wave ultraviolet radiation, weak red fluorescence under short-wave radiation, and a characteristic absorption spectrum. Its most unusual optical feature was a moderate to strong whitish blue transmission effect observed when intense light was directed through a small opening onto the stone. The phenomenon was also visible without magnification when the spinel was held a few inches from an incandescent or fluorescent source, while a faint bluish haze could be seen at arm’s length in sunlight. The effect was interpreted as probably resulting from light scattering by minute inclusions or dislocations, comparable to similar reactions in other gem materials. Microscopic examination showed that the stone was only slightly included. Parallel rows of tiny octahedra, commonly encountered in spinels from classic Myanmar and Sri Lankan localities, were conspicuously absent. Instead, the dominant internal features were numerous sheets of polysynthetic twinning intersecting in two directions throughout the gem. Coarse needles or growth tubes were associated with these planes in several areas. Immersion in methylene iodide revealed faint color zoning apparently aligned with some of the twinning features. Densely packed clouds of minute particles occurred through much of the stone, appeared to represent exsolutions, and were oriented in three directions. A few small, thin, transparent, nearly colorless tabular euhedral crystals were also present. The combination of physical and optical properties, color, and visual appearance matched spinels attributed to a newly reported Tanzanian deposit that the author had examined during a visit to Idar-Oberstein. Tanzanian spinel was also available at the 1990 Tucson Show. The note documents distinctive optical and microscopic characteristics of this specimen while retaining its Tanzanian origin as a reported attribution.