Pollucite

Crystal system Cubic
Transparency Transparent to translucent
Luster Vitreous
Fracture Conchoidal
Cleavage Perfect, distinct
Specific Gravity 2.92
Hardness 6.5
Optical Character Single Refractive
Refractive index 1.520
Birefringence 0.012
Dispersion
Fluorescence Orange to pink
Pleochroism
Chemical Formula H2Cs4Al4(SiO3)9
Comments Attacked by acids
Streak white

This is another of the rare minerals that are occasionally cut for collectors; however, since it is either colorless or light gray and has virtually no dispersion, it can hardly be classed as more than a curio. The name is derived from "Pollux", the name of a god in Greek mythology, and is pronounced po-Loo-site.

Pollucite is a caesium-aluminum silicate, corresponding to the formula H2Cs4Al4(SiO3)9. It crystallizes in the isometric system, sometimes in cubes, but may also be massive. The only known localities are on the Isle of Elba and near Hebron and Rumford, Maine. The hardness is 6 1/2, the R.I. varies from about 1.517 to 1.525, the luster is vitreous, and the S.G. ranges from 2.90 to 2.94.