Pumice stone is mainly used to separate out oxygen from Nitric acid.
Pumice stone is a volcanic rock and can be withstand high temperature. When Nitric acid is to be decomposed, it is made to fall dropwise on heated pumice stone, where nitric acid's thin layer gets sufficient kinetic energy for its decomposition.
\(4HNO_3 \to 4NO_2 + 2NH_2O + O_2\)