Zircon has been around since antiquity. Where it's name comes from is uncertain. It has a great brillance and intense fire due to its high refractive index and strong dispersion. It is sensitive to knocks and pressure, and edges are easily damaged. It contains a relatively high content of radioactive elements (uranium, thorium) causing large variation of pysical properities. A normal zircon (called high zircon) is one which has the highest values in optical properities. Ones with lower values are called low zircons.