Pumice is a relatively light weight volcanic, porous, stone material that is popular on the west coast to provide aeration to soil mixes. Because it is a stone material, it does not break down in the soil. Its porous nature allows it to suck up and hold onto moisture, while permitting excess water to flow through the mix easily.
In reality, it is much like perlite, in that perlite is similarly created by subjecting a volcanic stone to heat, which causes the trace water contained in the perlite to expand, "popping" the perlite like popcorn. Some people object to the white appearance of perlite, and prefer the grey appearance of pumice.
Pumice is an alternative in a soil mix to rice hulls. Some prefer the pumice as rice hulls will eventually break down. However, in our own observation this takes at least 4-5 years to occur. And considering that when rice hulls break down, they release their significant silica content to the surrounding soil, then there are obvious advantages to both materials in a soil mix.
Most soil mixes aim for approximately 33% aeration from pumice, perlite, vermiculite or rice hulls.
Equivalency Measurement- 1 cu ft of pumice weighs approximately 55 lbs