Malawi Ruby and Sapphire: Gemstone Information
While Malawi Ruby has been known at least since 1958 when the Nyasaland Geological Survey reported on it, regular mining of the deposits at Chimwadzulu Hill is more recent. The area is 145 km south-east of the capital city, Lilongwe. The area comprises mainly metamorphosed ultramafic rocks with an exposed surface and an exposed surface diameter of about 1 km. The surrounding county rocks are Precambrian metasediments metamorphosed into schists and gneisses. The bedrock is deeply weathered on the hill with a surface layer of iron-rich red porous clay.
Corundum is found in the topsoil and there is good evidence of bedrock resources. Minerals in aggregate found with the corundum include quartz, magnetite, hematite and chromite and separation work is necessary for extraction of corundum. David Hargreaves, Chairman and CEO of Minex (Pvt.) Ltd and the owner and operator of the mine, who has supplied me [MO’D] with details of its operation, reports that more than 50% of the corundum is magnetic and this property may be found useful in separation. Corundum-bearing soil is scraped off the bedrock and sent for processing.
When Minex began developing the mine in 1995 about 10% of the orebody had been worked. Recovery potential seems promising. The area had been described as a sapphire deposit in which the occasional ruby was found. Naturally coloured sapphire crystals of quality were rare and seemed excellent candidates for heat treatment. Conventional heat-treatment methods, however, did not achieve good colour. Fortunately the discovery of ruby in natural colours ranging from pink to purple with some orange padparadschah helped the operation to survive. Further experimental work on heating the sapphires has now produced some good blue, yellow and fancy colours though no red stone were achieved, nor could naturally red crystals be enhanced. Taking into account the high probability that the gem-buying public will turn to stones whose colour can be shown to be natural, the potential of Chimwadzulu corundum is good. Chimwadzulu corundum can be described as ranging from a true pink
to a very dark purplish blue. Stones weighing more than 1 ct sell on their individual merits though melee, high in volume and with sales vital, needs colour consistency. For this reason stones have been grouped into nine categories; ruby, padparadschah, purple, pink, yellow, teal, blue, cognac and fancy.
Columbia Gem House in the US has distributed the Malawi corundum and the profile is still expanding. Locality information is always useful when selling gemstones. A paper by Professor Andy Rankin in The Journal of Gemmology in 2003 described the Chimwadzulu material and the results of heating trails. The ratios (wt% oxides) of Cr/Ga vs Fe/Cr for Chimwadzulu and Thai and Kampuchean material shows only a minor overlap: there is no overlap with Mogok, Mong Hsu, Vatomandry and Longido.