252                               PRECIOUS STONES. its dichroism; it is harder too. Olivine (Chrysolite) and Beryl (Emerald) are both lighter than Demantoid and are both doubly refracting, though Olivine is only feebly dichroic. Zircon (Hyacinth) is denser and harder than Hessonite and has a much higher lustre. Glass imitations in all cases are softer.