Prehnite is a hydrated calcium aluminum silicate that occurs in a soft yellow-green to apple-green color. Many specimens have an intriguing luminous quality. It has a vitreous to mother-of-pearl luster. Prehnite is a medium-hard gem with a rating of 6 to 6.5 on the Mohs scale. Most prehnite is translucent but transparent specimens are sometimes found. Prehnite has a density or specific gravity of 2.82-2.94 and a refractive index of 1.611-1.669.