Selected literature data and new results from our own spectroscopic studies using optical absorption, paramagnetic resonance, and Raman spectroscopy are used to give an overview of recent understanding of the coloration of natural zircon. Pure ZrSiO4 is colorless. Coloration is mainly related to point defects of the electron or hole type, although charge transfer mechanisms and crystal field-dependent internal electron transitions in f- and d-elements may sometimes also play a significant role.
There is a common substitution of tetravalent zirconium by trivalent rare earth elements (REE) and yttrium which is frequently not compensated by phosphorus or other elements. As a result, hole defects are the dominant type of coloration centers in reddish and some rose natural zircon crystals. On the other hand, electron centers may be involved in coloration of common yellowish-brownish zircon which displays low REE and yttrium contents. These two coloration mechanisms are the most common in natural zircon.
An unspecific absorption increasing steadily from the red to the blue part of the optical absorption spectrum may additionally affect zircon coloration. Color shades of grey, rose, and brown appearing with increasing intensity of this absorption feature may result. This optical characteristic, which is frequently observed in some zircons, is obviously related to the effects of radioactive irradiation and occurs in various types of zircon depending on their uranium and thorium contents.
Sharp absorption lines caused by internal electron transitions of the f-f type related to U4+ or, less typically, to Er3+ and Nd3+ substituting for Zr4+, occur frequently in the optical absorption spectra of zircon. As a rule, they have no significant influence on the visible color. However, very high U4+ or Er3+ contents combined with strong absorption in the blue part of the spectrum may cause greenish coloration. Rarely, green zircon coloration also occurs as a result of various broad absorption bands located in the red part of the spectrum.
Very high REE contents in some zircons are the most likely cause for a strong, absorption edge-like feature in the blue part of the optical absorption spectrum, resulting in a rarely observed intense yellow coloration. Blue zircon coloration is mostly artificially induced by heating, but sometimes also occurs in nature. Besides the increase in unspecific absorption causing greyish to brownish colors, there are no simple relationships between zircon coloration, radioactivity, and structural order (or degree of metamictization) of the crystal grains.