Zimbabwe, Rhodesia, gems

Gübelin examines emeralds from the newly discovered Sandawana deposits in the Belingwe area of present-day Zimbabwe, combining an account of their discovery and geological setting with measurements and microscopic observations intended to establish distinguishing characteristics. Laurence Contat and Cornelius Oosthuizen encountered emerald while prospecting for pegmatite minerals, with an initial discovery in October 1955 followed by a significant deposit found in May 1957. The emeralds occur in association with tremolite schists and pegmatite dikes in the Mweza Range, a setting compared with that of emeralds from Austria’s Habachtal. Geological knowledge remained preliminary, and the workings were still limited, although emerald-bearing material derived from erosion was also recognized. An initial lot of ninety-two emeralds was submitted for investigation. The faceted specimens described are small, weighing 0.15–0.38 carat, but display vivid, comparatively uniform green color with a warm yellow component that remains strong even at these sizes. A reported chemical analysis includes chromium, iron, magnesium, sodium, and lithium in addition to the principal constituents of beryl. Measured optical properties are relatively high for emerald, with representative refractive indices of approximately 1.593 and 1.586 and birefringence near 0.007. Specific gravity ranges from 2.744 to 2.768. Most specimens show the familiar emerald absorption spectrum, a weak red appearance through the Chelsea filter, and distinct bluish-green and yellowish-green dichroic colors. They are inert under the long-wave and short-wave ultraviolet radiation used in the study, with ultraviolet transmission ending near 320 nm. Because these properties overlap those of emeralds from other deposits, Gübelin places particular emphasis on inclusions. The most characteristic assemblage consists of fine tremolite needles approaching actinolite in composition, occurring as short scattered rods and long, slender, sometimes curved fibers. Tiny disk-like fissures along some needles create the appearance of flakes pierced by rods. Compared with Habachtal material, Sandawana emeralds contain finer needles, less densely grouped, with short rods accompanying the longer fibers. Other observed inclusions include brown garnet, limonite coatings and fracture fillings, and rare altered plagioclase, magnetite, and hematite. Dust-like green or brown patches, described as splashes, may form halos around resorbed garnets; an illustration identifies such formations as comprising extremely minute liquid droplets. Additional fissures form interrupted lines and bands parallel to the basal plane, differing in appearance from the more extensive layers of disk-like fissures described in Russian emeralds. Gübelin concludes that the combination of physical properties, tremolite morphology, internal irregularities, and vivid color provides a practical basis for recognizing Sandawana emeralds, while emphasizing the importance of direct observational experience.