Prosopite: Gemstone Information
Prosopite is CaAl2(F,OH)8 and forms transparent small tabular colourless crystals of the monoclinic system. Crystals may become translucent on exposure and have one direction of perfect cleavage. The hardness is 4.5 and the SG 2.88–2.89. The RI for the alpha, beta and gamma rays is 1.500–1.501, 1.503–1.509, 1.509–1.510, biaxial positive with birefringence 0.009. Prosopite occurs in granite pegmatites, sometimes in base metal deposits or fluorine-rich granites and greisens. Prosopite from the Santa Rosa mine, Mazapil, Zacatecas, Mexico, occurs with azurite and can resemble turquoise though its higher RI adequately separates the two materials.