High-frequency spark discharges were applied to single-crystalline wafers of Ge, GaAs, and Si. The spark-processed (sp-) samples were characterized by photoluminescence (PL) and Raman measurements. Strong and stable luminescence with wavelengths centered at 416 and 525 nm was observed in sp-Ge and sp-Si layers, respectively, when excited with a 325 nm laser beam. A considerable blue shift of the PL (compared to the unsparked specimen) was also detected for sp-GaAs with an average peak wavelength around 500 nm. The Raman Shifts of the spark-processed materials indicate that nanocrystals were formed, having diameters of 3.5-4 nm for Si and about 6 nm for Ge. A correlation between the PL wavelengths, the nanocrystal sizes, and the different semiconductor materials has been established based on the effective-mass approximation. Making use of this model the nanocrystallite sizes have been found to range between similar to 3 nm for Si and similar to 5 nm for Ge. The related wavelengths for optical transitions confirm the PL results. The findings support the quantum confinement model.