corundum, sapphire, Nigeria.12, inclusions.5, gems

Kiefert and Schmetzer describe blue and yellow sapphires from a secondary weathering deposit in Kaduna Province, about 45 km northeast of Jos, Nigeria, based on examination of 38 rough crystals and 31 cut stones. The rough sapphires are tabular, barrel-shaped or prismatic and occur with quartz, kaolinite, goethite, hematite and gibbsite. Gemological properties are typical of corundum, with RI nε 1.759–1.761, nω 1.768–1.770 and SG 3.99–4.01. Spectroscopy shows that the blue coloration involves Fe³⁺, Fe²⁺/Ti⁴⁺ and Fe²⁺/Fe³⁺ absorptions, with Cr³⁺ present in some specimens, whereas the yellow sapphires show Fe³⁺ and Ti³⁺ features. Characteristic microscopic features include pronounced growth and color zoning, fissures along basal parting planes, negative crystals, pseudosecondary healed fissures and secondary inclusions in open fractures. Identified mineral inclusions include albite, uranpyrochlore and zircon, some associated with primary fluid inclusions.