Benitoite is a rare curiosity found almost exclusively in the great serpentine body of the New Idria mining district of central California. Its sapphire-blue color is unusual, but it really comes out in ultraviolet light, where it shines with a bright blue fluorescence.
Mineralogists seek out benitoite because it's the simplest of the ring silicates, with its molecular ring being composed of only three silica tetrahedra. (Beryl, the most familiar ring silicate, has a ring of six.) And its crystals are in the rare ditrigonal-bipyramidal symmetry class, their molecular arrangement displaying a triangle shape that geometrically is actually a bizarre inside-out hexagon (this is not correct technical crystallographic language, you understand).
Benitoite was discovered in 1907 and was later named the state gemstone of California.