今天小女兴起写了这段字。第一回 

Purchased this  “carborundum stone”. It is silicon carbide stone.

Carborundum, or silicon-carbide, runs 9.5 on the Mohs scale, where diamond, the hardest substance on earth, runs a 10. 66 HRC knife blades (the hardest I’ve seen) are about a 6.5 on the Mohs scale. Should be a very fast cutter for whatever grit it is.