The accepted definition of thermoluminescence is the thermal release of trapped electrons with accompanying optical transition. This interpretation presupposes an exposure to ionizing radiation in order to fill the electron traps. Other processes such as mechanical stress and chemical reaction can also serve to excite electrons, as observed in triboluminescence and chemiluminescence. It is proposed that some instances of luminescence during heating are produced by physical processes other than the release of previously trapped electrons. Evidence for this is provided by the correlation of certain glow-curve peaks with crystalline annealing, decomposition, and polymorphic transition. Measurements of annealing and polymorphic changes were made through x-ray diffraction. Attempts to interpret the thermal or radiation history of geological specimens by thermoluminescence must account for both radiation-induced and radiation-independent forms of light generation.