  Understanding the electrical properties of rocks is of fundamental interest.
We report on currents generated when stresses are applied. Loading the center
of gabbro tiles, 30x30x0.9 cm$^3$, across a 5 cm diameter piston, leads to
positive currents flowing from the center to the unstressed edges. Changing the
constant rate of loading over 5 orders of magnitude from 0.2 kPa/s to 20 MPa/s
produces positive currents, which start to flow already at low stress levels,
<5 MPa. The currents increase as long as stresses increase. At constant load
they flow for hours, days, even weeks and months, slowly decreasing with time.
When stresses are removed, they rapidly disappear but can be made to reappear
upon reloading. These currents are consistent with the stress-activation of
peroxy defects, such as O$_3$Si-OO-SiO$_3$, in the matrix of rock-forming
minerals. The peroxy break-up leads to positive holes h$^{\bullet}$, i.e.
electronic states associated with O$^-$ in a matrix of O$^{2-}$, plus
electrons, e'. Propagating along the upper edge of the valence band, the holes
are able to flow from stressed to unstressed rock, traveling fast and far by
way of a phonon-assisted electron hopping mechanism using energy levels at the
upper edge of the valence band. Impacting the tile center leads to
h$^{\bullet}$ pulses, 4-6 ms long, flowing outward at ~100 m/sec at a current
equivalent to 1-2 x 10$^9$ A/km$^3$. Electrons, trapped in the broken peroxy
bonds, are also mobile, but only within the stressed volume.
