Zircon: Zirconium silicate with the formula Zr(SiO)4. It crystallizes in the Tetragonal System as pyramidal prismatic crystals, often twinned, and occurs in gray, yellow, reddish-brown, and green colors. Heating can induce various colors. It often contains trace elements such as Uranium and Thorium, which over long periods cause radioactive decay that leads to the destruction of the crystal lattice. This altered form is called “low Zircon” or “metamict Zircon,” characterized by nearly isotropic material with a low refractive index. Untouched Zircon is referred to as “high Zircon.” Density varies from 3.90–4.10 g/cm³ for low Zircon up to 4.65–4.80 g/cm³ for high Zircon. Mohs hardness is 7.5. Refractive indices range from 1.78–1.85 for low Zircons to 1.92–1.94 and 1.97–2.01 for high Zircons.
