Is it permissible to deliberately roughen the place of prostration to make the mark of prostration appear on the face, in order to fulfill the description "Their mark is on their faces from the effect of prostration," and is this considered showing off?

The mark of prostration on the face is not the sign left by the roughness of the prayer mat, but rather a light, radiance, and humility that appears on the face due to abundant prayer and obedience. It is not prescribed to intentionally prostrate on a rough surface to leave a visible mark. If the intention behind this is to show off one's righteousness, then it falls under prohibited ostentation (riya').