Surfaces of diamond crystals are examined by coating the surfaces with thin metal films, launching laser beams to the diamond surfaces in a slanting angle, detecting defects and particles on the diamond surfaces by the scattering of beams and counting the defects and particles by a laser scanning surface defect detection apparatus. Diamond SAW devices should be made on the diamond films or bulks with the defect density less than 300 particles cm−2. Preferably, the diamond surfaces should have roughness less than Ra20 nm. Diamond SAW filters can be produced by depositing a piezoelectric film and making interdigital transducers on the low-defect density diamond crystals.