Diamond finishing instruments use diamond particles of size:

Diamond particle size is commonly categorized as coarse (125 to 150 µm), medium (88 to 125 µm), fine (60 to 74 µm), and very fine (38 to 44 µm) for diamond preparation instruments.  Diamond finishing instruments use even finer diamonds (10 to 38 µm) to produce relatively smooth surfaces for final finishing with diamond polishing pastes. Ref: Sturdevant's art and science of operative dentistry. 4th edition page 338