Recent research work is reported on cold cathode luminescence (CL) microscopy and CL microspectrophotometry of diamond produced by means of plasma-activated mixtures of hydrogen and methane gases. Diamond layers of octahedral, cubooctahedral or cubic habit on SiAlON substrates as well as ballas-type layers were investigated. Diamond was also deposited on sheet substrates of titanium, niobium, tantalum, chromium, molybdenum, tungsten, iron (spheroidal and lamellar cast iron), cobalt and nickel. Studying CL colors together with CL spectra has proved to be a reliable diagnostic method for differentiating between the morphological properties of CVD diamond on SiAlON substrates. It is however not possible with this method to clearly differentiate all diamond layers on any one of the sample substrates investigated. These findings could justify a future application of both CL microscopy and CL microspectrophotometry in production control.