New emission spectra for MgO and CaAl2SiO8 (glass) are observed from 430 to 820 nm. Taken with previous data, we suggst that transparent solids display three regimes of light emission upon shock compression to successively higher pressures: (1) characteristic radiation such as observed in MgO and previously in other minerals, (2) heterogeneous hot spot (greybody) radiation observed in CaAl2Si2O8 and previously in all transparent solids undergoing shock-induced phase transformations, and (3) blackbody emission observed in the high pressure phase regim in NaCl, SiO2, CaO, CaAl2Si2O8, and Mg2SiO4. The onset of regime (2) may delineate the onset of shock-induced polymorphism whereas the onset of region (3) delineates the Hugoniot pressure required to achieve local thermal equilibrium in the shocked solid. We also propose that the hot spot temperatures and corresponding shock pressures determined in regime (2) delineate points on the fusion curves of the high pressure phase.