Thicker layers appear more reddish in color.
Above: The disk with the star pattern is polished aluminum coated with pure diamond; all of the rest (the rim and the chain) is gold.
Below: A microphotograph of two polished silicon "coupons" coated with 24 micro-inches of pure diamond coating for testing wear resistance.
To simulate wear from "dirty" oil, for example, the samples were tumbled in a lapidary polishing barrel in a slurry of 1 mil (0.001 in) silicon carbide (SiC) grit for 120 hours.
The microphotograph was made with a camera and close-up lens. Samples were placed on a blue paper background with "cross-lighting" so that shiny surfaces appeared as dark, because they scattered none of the lighting up to the camera.
The link to the technical details tells how the production of pure diamond coatings proceeds in a cycle of steps in which soft carbon is deposited and then ions "pound" it into diamond-like carbon. Seen in the "New" photograph is unprocessed carbon left on the surface of the diamond coating. The bare silicon is black because it is polished and shiny at first. After 120 hours of erosive wear the bare silicon is ground away while the coated part has been protected. The unprocessed carbon has been burnished off and the underlying pure diamond is exposed. Being shiny, the pure diamond coating appears as black.
