The color in zircon (ZrSiO4) comes from both uranium ions and radiation damage. U4+ enters the zircon structure because, like Zr4+, it has a large ionic radius. U4+ substituting in zircon causes the blue color of heat-treated zircon. Over geologic time, the uranium undergoes radioactive decay and the resulting radiation damage centers cause a range of red-brown and amber colors. Heat treatment removes the radiation damage centers and restores the blue color with which zircon presumably originally crystallized. Green colors are associated with a mixture of the blue color from uranium and red-brown color from radiation damage centers. Highly radiation-damaged zircons are called metamict zircons. Colorless zircons have little uranium. 

Zircon Visible Spectra (400 - 1200 nm)

Link to an extensive collection of references to mostly color and visible spectroscopy of zircons

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last updated:22-Jan-2021