The acoustic‐emission rate is shown to be a direct measure of wear rate in a brittle solid and is used to study the strong dependence of drilling rates on the fluid environment. It is proposed that aggregation of ions in the drilling fluid produces charged or noncharged species which determine the surface zeta potential. The Rebinder‐Westwood environmental effects on mechanical properties correlate well with the zeta potential and it is shown that the effects also correlate well with the state of charge of ions and ion aggregates in the fluid environment.