Just a quick note on CrO.

The average particle size (e.g. 0.5um) does not fully describe the grit or scratching characteristics of CrO, which is definitely not the same as diamond at 0.5um.

Diamond is very aggressive at 0.25um, but the rounded shape of the CrO particle means the facets that actually scratch are very small. I think the range is closer to far less than 0.25um which produces such a slight refinement to the edge as to make CrO the defacto standard over diamond producing a very smooth and uniform surface. I have never found its equal on balsa or strops.

Enjoy.