Mambilla sapphire is the trade name for blue and parti-coloured corundum recovered from the alluvial gravels of the Mambilla Plateau in Taraba State, northeastern Nigeria, with commercial production from approximately 2011 onwards. The deposit is geologically a classic basaltic-origin source: Cenozoic alkali basalts hosting corundum xenocrysts, weathered and reworked into surface placer concentrations.

The colour profile reflects this geology. Mambilla sapphires are predominantly dark blue, often with a noticeably inky or blackish tonality, the result of high iron content as the dominant chromophore. Greenish-blue and teal hues occur, as do parti-coloured stones showing zoned colour distribution. The dark body and strong dichroism — blue along the c-axis, greenish-blue perpendicular — are diagnostic of basaltic-origin material and contrast with the vivid, cleaner blues produced by metamorphic deposits in Sri Lanka, Kashmir or Madagascar's central highlands.

Treatment

Standard heat treatment is essentially universal for material entering the international trade. Heating in oxidising conditions can lighten the dark inky body, improve clarity by dissolving silk inclusions, and develop more saleable medium-blue tonality. Untreated Mambilla sapphires are uncommon in the market; when offered, they typically sit at the darker end of the colour range and require origin-and-treatment laboratory documentation to support the claim.

Identification

Spectroscopic features include the iron-related absorption lines characteristic of basaltic sapphires, with relatively low chromium and gallium signatures. Inclusions reported in laboratory studies include feldspar, columbite-group minerals, zircon haloes and healed fissures with characteristic two-phase fluid inclusions. Origin determination at the major laboratories distinguishes basaltic from metamorphic origin reasonably well, but discriminating Mambilla from other basaltic sources such as Pailin (Cambodia), Australian Inverell or certain Thai deposits requires careful trace-element analysis and is not always conclusive. For the trade, Mambilla sapphire is best understood as a member of the African basaltic-origin family of sapphires rather than as a uniquely identifiable origin in the Kashmir-Burma sense.