chrysoberyl, alexandrite, gems

E.J. Gübelin describes the occurrence, optical properties, and inclusions of alexandrite from the Lake Manyara emerald deposit in Tanzania. The association of alexandrite, emerald, and ruby recalls deposits at Tokowaya in the Urals and Novello in present-day Zimbabwe. X-ray examination identified actinolite as the dominant mineral in the alexandrite host rock, accompanied by enstatite, iron-rich olivine, chondrodite, chrysotile, and dark spinel. Gübelin classifies the host as actinolite schist and relates alexandrite formation to the unusual contact between beryllium-bearing pegmatitic fluids and chromium-bearing rocks. Most specimens showed characteristic trilling, in which three intergrown crystals form a twin assemblage. Some clear pieces could yield faceted gems of up to five carats, although many crystals and fragments were etched, fractured, or heavily included. The material changed from bluish green in daylight to pronounced raspberry red under incandescent light. Representative refractive indices were 1.745, 1.750, and 1.754, with birefringence of 0.008–0.010 and a density of about 3.71 g/cm³. Electron-microprobe analysis of two crystals found chromium and iron, with no detectable vanadium. Gübelin attributes the color change to Cr³⁺ occupying two differently sized octahedral sites in chrysoberyl, whose overlapping absorption bands interact with the differing spectral distributions of daylight and incandescent light. He treats iron chiefly as a modifier of hue and a suppressor of luminescence. The Manyara specimens showed dull red fluorescence under long-wave ultraviolet light, weak yellowish fluorescence under short-wave ultraviolet light, and no response to X-rays. They were opaque to short-wave ultraviolet radiation, as were the natural and flux-grown synthetic comparison stones, while the Czochralski-grown material transmitted it. Inclusions included actinolite fibers, biotite flakes and books, and an apatite crystal in one specimen. Dense actinolite fibers could produce chatoyancy. Partly healed fractures contained liquid-filled tubes and branching channels, and some crystals showed layered or pseudohexagonal color zoning. Despite the strength of the color change in fine specimens, Gübelin expected the supply of gem-quality material to remain scarce and irregular.