Solidified lava used as an abrasive (6)
I believe the answer is:
pumice
'solidified lava used as an abrasive' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pumice that I've seen before include "Solidified lava used as an abrasive" , "Lava stone" , "Light form of solidified lava" , "Mineral" , "Volcanic stone used as abrasive" .)