Diamond/ carbon nanotubes have the property of being combustible, so perhaps they are not a good choices. I think crystals will be more resistant to erosion but can be fragile due to the cleavage planes.

Diamond/ carbon nanotubes have the property of being combustible, so perhaps they are not a good choices. I think quartz crystal might be more resistant to erosion than granite and supposedly does not have a big issue with fragility due to cleavage planes.