Is this is a fact or opinion diamonds are the hardest substance on earth?

Diamond mineral is rated on the Mohs Scale at 10, which is the highest/ hardest rating on the scale. It is the only mineral given that rating. the Mohs Scale is most useful as a field guide, where a geologist might use one mineral to mark another. This means that diamond can mark every other natural mineral.

The Mohs scale is not useful for industrial or man-made materials.