Details are given of an experimental study of the current-voltage characteristics and instability of the field electron emission from amorphous diamond films prepared using a magnetic field-filtered ion deposition technique. A switch-on phenomenon is observed, and two types of instability are found, i.e., fluctuation at low currents and an abrupt change in emission associated with local breakdown events at high currents. Finally, it is found that, in order to establish stable emission with a current density approaching 50 mA/cm(2), it is necessary to undergo the switch-on and local breakdown processes. These phenomena are characterized,: and the physical mechanisms responsible for them are proposed.