Rock, when fractured in the laboratory, emits electrical signals with a broad maximum in the power density spectrum in the band from 900 Hz to 5 kHz. Quartz-free basalt rocks radiate both light and low frequency electrical signals as intensely as quartz-bearing rocks, suggesting that the piezoelectric effect of quartz is at most a minor contributor to the total power radiated. Newly created rock surfaces acquire a local net charge distribution when exoelectrons are expelled and removed from the vicinity of the new surfaces by collisional and other processes. The rotational, vibrational, and linear motion of rock fragments with charged surfaces is the major contributor to the observed power density spectrum of the low-frequency electrical signals. Neither optical spectral lines nor high-frequency electromagnetic radiation, characteristic of electric discharges, occur at fracture. Exoelectron bombardment of the ambient fluid surrounding the sample, not electric discharge, is the excitation source for the light emitted at fracture. ¿American Geophysical Union 1987