Rubies and sapphires from winza, central tanzania

Abstract

Since late 2007, rubies and sapphires have been mined by hand methods from both eluvial and primary deposits at Winza in central Tanzania. The gem corundum is related to "dikes" of amphibolitic rocks that belong to the Paleoproterozoic Usagaran Belt. Based on crystal morphology, Winza corundum is subdivided into two types: prismatic-tabular-rhombohedral and dipyramidal. In general, medium red and dark (orangy) red top-quality rubies are rhombohedral. Pinkish red and purplish red rubies, as well as pink, purple, and blue (often strongly color zoned) sapphires are, for the most part, dipyramidal. The top-quality rubies are characterized by a distinct assemblage of long tube-, fiber-, needle-, or hair-like inclusions containing an orange-brown material (most likely limonite). The lower-quality material generally contains a larger amount of solid inclusions (mostly amphibole crystals), fissures, and growth features. Unique to corundum from this locality are bluish violet color zones oriented parallel to the prism and basal pinacoid, and occasionally also parallel to rhombohedral and dipyramidal faces. The relatively high Fe content of Winza rubies separates them from most other natural and almost all synthetic counterparts. © 2008 Gemological Institute of America.