Jeremejevite: Gemstone Information
Jeremejevite is Al6(BO3)5(F,OH)3 and some pale blue-green, cornflower blue, brown or colourless transparent stones are found. The crystals of the hexagonal system occur as hexagonal prisms, sometimes tapered with vicinal forms and with pyramidal terminations; sector zoning is characteristic. The hardness is 6.5–7.5 and the SG 3.28–3.29. The RI for the ordinary and extraordinary rays is 1.644–1.647 and 1.637 respectively.Jeremejevite, however, may be anomalously biaxial with the alpha, beta and gamma rays giving the RI 1.637, 1.644–1.647 and 1.644–1.652. In either case the optic sign is negative. The birefringence is 0.007–0.013.Jeremejevite is a rare late hydrothermal mineral formed in granite pegmatites: most facetable crystals come from Cape Cross, Swakopmund,Namibia. Large crystals have come from Mt Soktui, Nerschinsk district,Adun-Chilon Mountains, Siberia, Russia.