The formation of calcium, strontium and barium colloids both in heavily electron irradiated samples and in additively colored crystals of CaF/sub 2/, SrF/sub 2/ and BaF/sub 2/ has been investigated. Detailed data on the temperature dependence of the efficiency of colloid formation by irradiation have been obained. The growth of metallic particles in additively colored samples containing F and M centers has been studied for different color center concentrations and annealing temperatures. The optical absorption bands due to metallic colloids have been calculated using the theory of Mie. To take account of the pressure exerted by the matrix on the metallic particles several corrections to the optical constants of the metals have been introduced. A good agreement between theoretical calculations and experimental results has been obtained. The evolution of colloids along several thermal annealing experiments has also been investigated. A diffusion-limited model has been used which accounts for the dependence of the colloid radii with the annealing time.