Dear WW: I think it likely crystalline structure is basis
or reflectiveness. I have read that diamonds are formed
deep in volcanoes at tremendous temperature and pressure.
Industrial diamonds are now made synthetically, but I don't know if the synthetic ones can yet compete pricewise.

And remember, the cars of today run ten times as far as the
cars of the thirties, because the parts can now be machined
to tolerances not possible in the thirties, using diamond abrasives.