Bill Tindall, E.Tn.
I did make an undocumented assumption in referring to chromium oxide as a historical abrasive for metal abrasion relative to alumina. Small particle chromium oxide would be technologically easy to manufacture. No so with submicrom alumina. In referring to chromium oxide as "historical" relative to alumina I made the assumption that people figured out how to make and use chromium oxide as a metal abrasive before submicron alumina. If this assumption is in error enlighten me of the comparative dates for when these materials became used for polishing metal.

(submicron alumina abrasive is not made by simply crushing aluminum oxide and screening)