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.