Alexandrite is rare and usually low quality, although in the past few years some very excellent specimens of alexandrite from the Carnaiba mine appeared on the market. One pegmatite near Jaqueto yielded a 120,000 carat twinned crystal.
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In the region surrounding the Carnaiba deposit, Middle Proterozoic quartzites, phyllites, and quartz-chlorite schists are intruded by granite. Within this succession are tabular serpentinites and serpentine-talc schists covering an area of 10 to 15 km by 200 m that are surrounded by granite. Emerald occurs in the serpentinites adjacent to granite and also within a large xenolith in granite. Oligioclase-beryl-muscovite pegmatite and quartz veins with beryl, apatite, tourmaline and sulfides also are found within the ultramafics and less often within quartzite.
Biotite-phlogopite-schist and phlogopite-talc-schist which contain emerald and alexandrite lie near veins in tectonic zones within the ultramafics. The emeralds have Cr2O3 contents up to 0.73% and are disseminated in glimmerites and generally are 2 to 3 cm (0.8 to 1.2 in) in length. Some of the emeralds are high quality but mixed with considerable amounts of low-quality material.
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