The data for the maximum emission currents from needle-shaped emitters with differing diamond coatings were empirically analyzed. The coatings studied were chemical vapor deposition diamond, natural diamond, and nanodiamond. Two parameters were chosen to characterize the emissive properties: (1) the dependence of the maximum current (I-max) on the coating thickness (D), i.e., I(D)= DeltaI(max)/DeltaD and (2) the dependence of the threshold voltage V-th on [(D);DeltaV(th)/DeltaD]. The dependence of I-max(D) and I-max/V-max(D) were determined from the experimental data for the three different diamond coatings. The maximum current I,, is very different for these three different coatings and is also a function of the coating thickness, D. Both the maximum current and the transconductance of field emission tips can be increased significantly by diamond coatings. A strong, nearly linear, dependence of I-max on diamond thickness was found. An empirical estimate of the thermal conductivity of nanodiamond, based on the field emission data, gave 2.71 W/cm K. The maximum current output from multitip arrays was also analyzed and an optimization procedure was devised that suggested a route to "engineered coatings."