Summary: Almost 400 specimens of natural and synthetic quartz were γ-irradiated and then heated. Polarized absorption spectroscopy, EPR, and spectrochemical analysis were used to investigate the various colors produced.
A blue color originated from absorption by the A1 and A2 bands at 1·85 and 2·55 eV. The substitutional A1 EPR hole-type center usually considered to be the cause of the color in smoky quartz does not correlate with A1 and A2, but with A3, a previously unreported absorption band at 2·90 eV. The A1 and A2 absorptions bleach at temperatures as low as 280 °C and as high as 360 °C the range for A3 being 140 to 380 °C. The B band at 3·95 eV was previously reported only in irradiated fused silica.
Greenish-yellow colors (appearing yellow when of low intensity) were observed after irradiation (followed by heating in some instances) in many specimens of natural and synthetic quartz; the color originates from the tail of a strong absorption band in the ultraviolet. The bleaching temperature range is the same as that for A2.
The wide variation in color of natural and irradiated smoky quartz can be explained by combinations of the smoky (A3), blue (A1 and A2), and greenish yellow absorption features.