What does Pumice Stones mean?

Volcanic stone used for stone washing garments, pumice stones are popular due to its strength and light weight.

Heddels explains Pumice Stones

Because of its rough, porous surface, pumice is especially useful as an abrasive. During the stone washing process, pumice stones work like sandpaper to abrade the jeans, removing some of the dye particles and enhancing their worn, faded look.

Raw denim garments are untouched by pumice stones.

Additional Resources

A single pumice stone:

Source: backpack-newzealand.com