Pumice, like perlite, is a product of volcanic activity and mined from quarries, ground and sieved for aggregate size. It is commonly found in areas with high volcanic activity is used similar to perlite to lighten potting mixes.
Pumice is re-usable and porous so it drains quite well. Typically pumice contains some silica which can become plant available over time.
Based on the source of volcanic rock, pumice can contain high levels of sodium that must be leached out prior to use. Pumice has very low water holding capacity due to the large size of the pores formed during the production process. Also, due to the nature of the material used to form pumice, there is almost no cation exchange capacity to hold nutrients for plant uptake as needed.