beryl, emerald, Rhodesia, Sandawana, Zimbabwe, gems

E.J. Gübelin characterizes emeralds from the recently discovered Sandawana deposits in Southern Rhodesia [Zimbabwe], combining an account of their discovery and geological setting with chemical, optical, and microscopic examination intended to establish distinguishing features of origin. Prospectors Laurence Gontat and Cornelius Oosthuizen found the first emeralds in October 1956, followed seven months later by a richer occurrence. The deposits lie on the southern side of the Mweza Range, where pegmatite dykes intersect tremolite schists within a complex assemblage of ancient metamorphic and intrusive rocks. Gübelin interprets emerald formation as a product of contact metamorphism bringing beryllium-bearing granitic material into interaction with chromium-bearing rocks derived from ultrabasic intrusions. Study of ninety-two cut stones reveals a vivid emerald-green colour with a yellow glint, retained even in very small gems. Fracturing and small crystal size limit most available cut stones to less than a quarter carat. Chemical and spectrographic analyses indicate appreciable magnesium and sodium, together with chromium, iron, and lithium, although the small amount of material available restricts analytical accuracy. Detailed physical measurements on ten selected stones establish comparatively high refractive indices, with representative values near 1.593 and 1.586 and birefringence of approximately 0.007. Specific gravity is likewise relatively high, averaging about 2.756. The emeralds show yellow-green and bluish-green dichroism, a weak red response through the Chelsea filter, and the usual chromium-related absorption features. They are inert under both long- and short-wave ultraviolet illumination but display pale-red fluorescence in a crossed-filter fluoroscope; ultraviolet transmission ends sharply near 320 nm. Gübelin emphasizes that overlap in physical constants among emerald sources makes these measurements insufficient for reliable provenance determination by themselves. Internal characteristics provide stronger evidence, particularly short tremolite pins and exceptionally fine, sometimes curved fibres occurring singly or in bundles. Their morphology and distribution are compared with amphibole inclusions in Habachtal emeralds. Additional features include partly resorbed garnets with brownish halos, limonite-filled cracks, rare hematite, feldspar and magnetite inclusions, dustlike patches, and distinctive systems of fine fissures parallel to the basal plane. Lace-like residual liquid films observed in one stone are recorded without being treated as an established locality characteristic. Photomicrographs and comparative tables support the conclusion that Sandawana emeralds combine attractive colour with a recognizable suite of internal features useful in distinguishing them from other contemporary sources.