Pumice is created when super-heated, highly pressurized rock is violently ejected from a volcano. It is commonly used as an aeration amendment in soils. I prefer it to perlite because it doesn't break down as quickly and is also heavier, meaning it's less likely to float to the surface over time from repeated watering.
If ordering the cubic ft bag size, the pumice we stock is medium. If you are looking for pumice by the yard or pallet quantities on bagged pumice, then it's available in fine, medium, or large sizes.
NOTE: In our soils, we use pumice at a rate of 33-37% of our total media and a combination of 1/3 large pumice and 2/3 medium pumice.