Rupture and particle velocities were measured for stick-slip events occurring during frictional sliding on surfaces composed of Westerly granite and Twin Sisters dunite and on a combination of the two rocks. The rupture velocities observed for stick-slip events on sliding surfaces of Westerly granite typically had values of 2--3 km/s. A few rupture speeds greater than the shear wave velocity vs were measured, but these values could be artificially high, since the times of rupture arrival may have been measured in a direction oblique to the direction of rupture propagation. Rupture velocities observed from stick-slip events on surfaces of Twin Sisters dunite also had values of 2--3 km/s, although lower velocities were occasionally measured. Rupture speeds on frictional surfaces composed of granite sliding in dunite ranged from about 1 km/s to the S wave speed of the dunite (4.6 km/s). Particle velocities were measured for stick-slip events on a similar suite of sliding surfaces. Compared to Westerly granite, stick-slip events on Twin Sisters dunite sliding surfaces are characterized by smaller stress drops Δ&tgr; and lower particle velocities. Stick-slip motion takes place in a nearly constant time when mass and stiffness of the loading system are fixed. This explains the observed linear relations between Δ&tgr; and particle velocity. A simple harmonic oscillator is a good model for the friction machine, predicting rise times and displacement time functions which are in good agreement with observations.