gems, euclase

E.J. Gübelin describes a sapphire-blue variety of euclase from the Miami district near Karoi, Zimbabwe, then Rhodesia. Previously reported euclase from the district was colorless, pale blue, or light green. The dark blue material was traced to a locality about 5 miles southwest of Miami, near the southwestern edge of a beryl- and muscovite-bearing gneiss shield. Gübelin proposed that euclase formed as a secondary mineral when beryl broke down during the cooling of the region’s pegmatites, at approximately 400–600 °C. Of the 33 rectangular and square faceted stones examined, only a few appeared uniformly dark blue. Most showed alternating dark blue and colorless zones. Electron microprobe measurements gave iron contents equivalent to 0.12% Fe₂O₃ in the dark blue portions and 0.06% in the colorless portions. Gübelin attributed the sapphire-blue color to ferric iron and suggested that color centers might also contribute. Spectrophotometry showed an absorption minimum at 465 nm and a slight shoulder near 570 nm. The specimens were inert under both long-wave and short-wave ultraviolet light. Measurements on the 12 largest stones gave refractive indices spanning 1.652–1.672 and a consistent birefringence of 0.019. The stones showed pronounced pleochroism in azure blue, Prussian blue, and greenish blue. The reported hardness was 7½ and the specific gravity ranged from 3.06 to 3.13. X-ray diffraction data from five Gandolfi patterns agreed closely with published values for monoclinic euclase and confirmed the identification. Under magnification, even apparently uniform blue stones showed cloudy or zoned color distributions resembling those of some blue sapphires. The colorless portions were nearly inclusion-free, while blue portions commonly contained fractures, including partially healed cracks with irregular liquid inclusions and open cracks that produced bright reflections as the stones moved. Gübelin observed no mineral inclusions and found the fracture patterns insufficient to establish the material’s identity or geographic origin. The resemblance to sapphire underscored the importance of optical and crystallographic measurements in identifying the new color variety.