Pumice is solidified foam that forms from lava rich in gases. It has numerous pores in the form of bubbles and irregular cavities that absorb moisture. This lightweight stone is used to amend garden soil improving aeration and drainage while maintaining moisture levels. 3/8" x 0 sized Pumice 1/4" minus with about 20 % fines. Our pumice is horticulture grade, pH neutral, and available in 2 sizes.