Zircon is formed by a compound of the metals silicon and zirconium with oxygen It has the strongest refraction of light after diamond. Zircon is found in Ceylon, Australia, Siam and South Africa. The colours of zircon range from colourless, yellow, yellow-red, red, blue, light brown to brown. For jewellery purposes, mostly colourless, yellow, light brown or brown stones are used. The colour of zircon can be changed by firing or radium irradiation. It often happens that zircons that have been given a different colour by irradiation change their colour again when heated or in daylight. The colourless zircon is often confused with rock crystal.