Quartz crystals of various geologic origins have been irradiated by Co⁶⁰ γ-rays so that to elucidate the relation between absorption coefficients and the contents of trace elements, especially of Al, Li and K. The absorption maxima perpendicular to the optic axis are at 480nm for natural smoky quartz, 460nm for artificially coloured smoky quartz. A clear correlation was noticed between absorption coefficients (a₂-a₁) and Al/Li ratio, suggesting that the colouring of smoky quartz is due to Al-Li defects. It was also noticed that K ion plays a negative role in the colouring. As a general tendency, it was noticed that quartz crystals formed under higher temperature conditions, such as those from pegmatites are more readily coloured than those from epithermal veins.