Light porous acid volcanic rock used for scouring (6)

I believe the answer is:

pumice

(Other definitions for pumice that I've seen before include "Piece of volcanic rock used as an abrasive" , "Light porous stone used as polishing powder" , "Light form of solidified lava" , "Light, porous volcanic rock" , "Porous lava" .)

I've seen this clue in The Mirror.

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