The thermoluminescence (TL) of ruby has some unique peculiarities:Ruby has only one TL peak after irradiation at room temperature while even nominally pure sapphire crystals usually have four or five TL peaks. The temperature of maximum and the intensity of this peak decrease with increasing chromium concentration. Irradiated ruby crystals have a "monotonic" TL that appears from the very beginning of the heating cycle, yet the peak temperature is quite high (∼350°C). Normally, the glow of crystals during routine TL processes disappears immediately (with the time constant of the registered set-up) after the shutdown of the oven. The ruby crystals have some kind of "afterglow" with two stages. The lifetime of the longer of them is about 14 min. Most importantly, a rise in luminescence at constant temperatures is observed some days after termination of the irradiation. The usual decay of luminescence is observed after the initial ionizing irradiation of the crystal. After a few days, isothermal conditions above the temperature of irradiation prompted a rise in luminescence which continued for as long as 5 h. All these peculiarities can be explained by one phenomenon: the transition of Cr4+ ions which appear in the crystal after irradiation into a Big Radius State at increased temperature. The activation energy of the transition is 0.2 eV. The radius of this state is about 50 lattice constants.