The fabrication of gated diamond field-emission cathodes is described and a theory of their operation is discussed. These cathodes are made using commercial diamond grit with the addition of Ni and Ca salts to enhance emission. The resulting structure resembles a field-emission Spindt cathode with the internal metal cone replaced by a similar to 100 nm layer of diamond grit. Emission from these cathodes occurs at the lowest reported gate voltage of any field emission device and is unaffected by operation at pressures of over 100 Pa of N-2. Operation in oxygen and H2S at pressures of 6x10(-4) Pa degrades emission, but the cathodes recover once the ambient pressure is reduced to below 1x10(-4) Pa. The emission current noise is 2.5% rms over an 8 h period and 1% rms over 3 ms. These cathodes suffer from high gate current that varies from 0.2 to 1000 times the emitted current. The high gate current is known to be process dependent and not inherent to the cathodes. The emission performance is explained by the stable negative electron affinity of diamond, which allows for injection of electrons from diamond into vacuum with little to no electric field, 0-1 V mu(-1). Cathode operation is limited by the injection of electrons into the diamond at the back metal-diamond interface, which depends upon the doping of the diamond and the roughness of that interface.