@wikipedia
Natural Gamma Ray emission from subsurface rocks
Table 1. Typical GR values for popular mineralsRock TypeGR, GAPI1Halite (Rock Salt)02Coal03Limestone5 – 104Sandstone10 – 205Dolomite10 – 206Shale80 – 1407Mica100 – 1708Sylvite (KCl)500