corundum, Malawi.12, gems

An unusual form of “silk” is described in sapphires offered on the London market and reported to originate from Chimwadzulu near Blantyre, Malawi, rather than the Umba River district of northern Tanzania, with whose material they had initially been associated. Parcels encountered over approximately a decade consisted chiefly of rather washed-out blue stones, often lacking brilliance, together with some finer examples, fancy colors, and color-change specimens. Their inclusions and fractures generally resembled those in Tanzanian sapphires. Heating to improve color was suspected as a possible cause of some fractures, but was not established. A small number of stones contained conspicuous, coarse silk differing markedly from the familiar rutile needles of corundum. Under magnification, the features appeared as extremely thin, apparently hollow cavities arranged in straight lines, frequently intersecting at nearly right angles. Some were sufficiently thin to produce Newtonian interference colors in reflected light. Angular relationships were investigated by photographing one specimen through its table facet, preparing a negative print, extending the inclusion traces, and measuring their intersections with an ordinary protractor. The predominant intersections were approximately 87° and 93°, broadly approaching the angle between corundum rhombohedral faces; another group approached the angle between a rhombohedral face and the basal pinacoid. Nevertheless, recorded angles ranged widely, from about 4° to 161°, and individual cavities included narrow wedges as well as broader angular forms. The author acknowledges substantial measurement limitations arising from the width of the traces, approximate determination of their centers, and the inclination of some inclusions away from the principal observed plane. These uncertainties do not, in his assessment, explain the departure from the usual 120° relationships associated with corundum silk. Comparable features were observed in approximately six stones, most showing mainly the nearly perpendicular intersections. A possible relationship to normal sapphire parting directions is suggested, but neither the linear arrangement nor the varied cavity shapes is explained. The observations document an unusual inclusion pattern, interpreted as exsolution cavities, while leaving its formation and crystallographic controls unresolved.