Plateau State is a mining region in central Nigeria centred on the Jos Plateau, where alluvial workings have produced sapphire, aquamarine, topaz, tourmaline, and zircon for the international coloured-stone trade since the late twentieth century. The state is one of Nigeria's most productive coloured-stone districts, a federal entity whose tin-mining heritage from the colonial period left a network of pits, washings, and prospectors' tracks that artisanal gem miners have since converted to gem production. Material from Plateau State is documented in Gems & Gemology and is encountered with regularity in Bangkok, Jaipur, and Idar-Oberstein.

Geological setting

The Jos Plateau is a granitic upland in central Nigeria built largely on the Younger Granites of Jurassic age, a suite of ring-complex intrusions that host pegmatites, alkaline volcanics, and the residual placers derived from their weathering. The combination of long-stable cratonic crust, deep tropical weathering, and abundant alluvial concentration is the same recipe that has made East African gem fields productive, and the Nigerian deposits share many gemmological characteristics with their Tanzanian and Kenyan counterparts.

Most gem production in Plateau State is alluvial: stones are recovered from gravels in stream beds, palaeo-channels, and reworked tin-mine tailings rather than from primary host rock. Mining is artisanal and small-scale, with diggers working pits by hand and washing concentrate in screens and sluices.

Sapphire from Plateau State and Mambilla

Nigerian sapphire from Plateau State and the adjoining Mambilla Plateau ranges across blue, green, yellow, and parti-coloured material, with the most desirable stones showing a clean medium blue comparable to commercial Australian or Thai-Cambodian goods. Iron-rich basaltic chemistry produces the typical inky-blue tone of unheated rough; routine heat treatment is used to lighten and clarify many production stones. Crystal sizes are modest by world standards, with faceted goods most often in the one-to-five-carat range and larger clean stones unusual.

Mambilla, in Taraba State just south-east of Plateau, is sometimes grouped with Plateau State in trade discussion because the geological setting and mining methods are continuous across the highland region.

Aquamarine, topaz, and other species

Aquamarine from Plateau State is found in pegmatitic settings on the plateau and in adjoining areas. Colour ranges from pale to medium blue, with some Santa Maria-style saturated material reported. Topaz, tourmaline, and zircon round out the production, with some material entering the imitation-emerald and treated-blue-topaz pipelines as commercial rough.

In the trade

Nigerian rough is bought principally by Thai and Indian dealers who travel to local buying centres around Jos, with finished goods reaching the international market through Bangkok and Jaipur. Production is modest compared to East African sources such as Madagascar, Tanzania, and Mozambique, and Plateau State material is more often encountered as commercial rather than fine goods. Origin determination by laboratory is possible for some stones but is rarely commissioned for the price points at which Nigerian sapphire trades.

Buyers should treat the locality designation conservatively — Nigerian sapphire is the more honest descriptor unless laboratory documentation supports a more specific attribution.

Further reading

• GIA Gems & Gemology — Nigerian sapphire and aquamarine field reports
• ICA — coloured stone origin and trade
• International Gem Society — sapphire localities