HandAmerican advertise leathers with high silica content. I am not sure whether that is a natural or an artificial "ingredient" of leather. That might account for the abrasive qualities of some leathers.

Silica is also considered to be the abrasive agent of the linen strop:

"And he mentioned something about the superior abrasive quality of linen over cotton in the manufacture of canvas strops. This may help clarify the intended purpose of using the linen when stropping."

The latter quote I took from the first post of the thread Illinois Razor Strop Company started by Scott (Honedright).