Nearly all limestones emit light when heated to a temperature below that incandescence. This thermoluminescence is a natural property of the calcium and magnesium carbonate minerals in the rock. It can also be induced artificially by irradiating specimens with gamma rays or X-rays. The natural thermoluminescence has been produced by radiation from radioactive impurities in the rock.

The light emission is measured by a photomultiplier tube circuit as the temperature of the specimen is increased. Both of these variables are recorded simultaneously by means of a two-point potentiometer recorder. A plot of the light intensity as a function of temperature is termed the "glow curve." This shows a series of light-intensity maxima which depend on the amount of radiation that the minerals have received, their impurity content, and their crystallization history.

The glow curves of the artificially induced thermoluminescence of many different limestones have been measured, and the shapes of these curves have been found to be characteristic of the limestone stratum from which the specimens were obtained.

Preliminary tests of the use of this property as a tool in surface correlation of limestones have shown it to be applicable under closely controlled sampling in thick limestones. It has also been indicated that certain well known Pennsylvanian limestones can be differentiated over a county-wide area by the amounts of thermoluminescence as measured from their glow curves.