The absorption spectra of fifty-two natural diamonds have been measured at 80 and 290 degrees K. Type I diamonds are shown to have two kinds of absorption centres which are not detected in type II diamonds. Differences in the infra-red absorption spectra between type IIa and type IIb diamonds are reported. The classification of diamonds is discussed. The absorption produced by irradiating diamonds with electrons, neutrons and $\gamma $-rays is described. The general nature of the changes produced by the different forms of radiation is the same, but significant differences in the breadths and wavelengths of the induced absorption lines have been observed. Electron-irradiated type IIa diamonds show absorption lines not observed in irradiated type I diamonds. There is a similarity between the sets of natural absorption lines in type I diamonds and those produced by irradiation. A model relating the induced absorption to interstitial carbon atoms, vacant atomic sites and regions of amorphous or graphitic carbon is proposed.