corundum, ruby, sapphire, Malawi.12, geology.11, gems

Rutland describes gem-quality corundum from Chimwadzulu Hill in southern Malawi, where mining began in 1965 following earlier geological investigation. The corundum occurs in situ in epidotized amphibolite and in residual and eluvial concentrations produced by weathering; rounded crystals also occur in fine-grained granulary matrix within a dike-like body. The crystals are generally rhombohedral, commonly with basal parting and triangular growth figures, and range in color from pink through yellow and pale green to dark greyish green, with some showing pronounced color zoning. Cut stones examined included blue, pale bluish green, and red specimens, with refractive indices of approximately 1.760–1.770 and densities of 3.88–4.13. Dichroism, absorption spectra, and X-ray fluorescence analyses are reported; iron was prominent in the blue stones, while red material contained chromium. The stones showed no fluorescence under long-wave ultraviolet light, while some fluoresced under short-wave radiation. Rutland also reports that parcels cut in London produced attractive stones, some up to about 12 carats, although many contained veils and sharp internal flaws; the finest colors were described as a very fine rich blue and a dark iron blue, with pinks much smaller and red stones pale but of very good color reminiscent of good Siam material.