Point analyses of natural calcite cements by SIMS confirm the earlier suggestions that nearly equal 20 ppm Mn is required to activate luminescence, whereas Fe concentrations in excess of nearly equal 1400 ppm quench it partly, independently of the Mn content. Below this Fe threshold, luminescence is bright at Mn concentrations in excess of nearly equal 225 ppm. In the range 20-225 Mn, all types of luminescence--dull, luminescent, and nonluminescent--can occur. This undifferentiated domain is evident also from a compilation of literature data. The type of luminescence is not a simple function of Fe and Mn concentrations; other parameters influence CL. Since most diagenetic calcite cements have Fe and Mn concentrations that place them within the field of mixed cathodoluminescence, these results force reassessment of models that equate the type of luminescence with the redox state of the diagenetic environment.