Neptunite: Gemstone Information
Neptunite is KNa2Li(Fe2,Mn2)2Ti2Si8O24 and forms opaque black or very dark red monoclinic prismatic crystals with square cross-sections and one direction of perfect cleavage; crystals show piezoelectricity. The hardness is 5–6 and the SG 3.19–3.23. The RI for the alpha, beta and gamma rays is 1.692, 1.702 and 1.734, biaxial positive with birefringence 0.029–0.045. Neptunite occurs in natrolite veins cutting a glaucophane schist in a serpentinite body at the Gem mine and Mina Numero Uno,San Benito County, California, USA.