With a polishing stone (books then and now feet) (6)
I believe the answer is:
pumice
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for pumice that I've seen before include "Piece of volcanic rock used as an abrasive" , "Solidified lava used as an abrasive" , "Light, porous volcanic rock" , "Light porous stone used as polishing powder" , "Volcanic stone used as abrasive" .)