A review is given of the present state of the interpretation of the yellow, violet and blue colours of natural rock salt as due to radiation. The yellow salt from Hall in the Tyrol and from Hallstatt is certainly coloured amicroscopically by radiation. The author has for a long time claimed the existence of an amicroscopic blue and violet colour in irradiated rock salt and this seems to be confirmed by the discovery of the R-centres ( MOLNAR, SEITZ). Many natural violet samples of rock salt are also optically empty and seemingly coloured by R-centres. According to a thorough investigation by L. WIENINGER, the following groups of coloured rock salt can be distinguished : salt coloured only by amicroscopic centres, only by colloidal particles and by centres and colloidal particles simultaneously. As sources of radiation the following must be taken into consideration: the uranium-radium-contem of rock salt, its potassium-content and the extinct activity from radioactive thermal waters, according to O. HAHN, which can be deduced from the helium-content of rock salt. Also taking into consideration the fact that only a fraction of the available radiation is used for colouring, it can be shown that all these sources suffice for the explanation of the amicroscopic yellow and violet colour, the two last named also for that of the colloidal blue colour. That not all rock salt is coloured in Nature points to the necessity of sensibilizing and stabilizing impurities, which may also promote the formation of colloids.