Search:

The brown stuff, Fe²0³, also known as hematite has a hardness value of 5 to 6 on the Mohs scale. It is used in jewelers rouge.
The black stuff, Fe³04, also known as magnetide has a hardness value of...

Apart from possibly being ever so very slightly abrasive, a good linen (or canvas) protects the fine leather surface of your strop. Every now and then, someone looking to edges with magnification...