pomice - Calcareous-free volcanic mineral material, the result of the "gaseous explosive" part of the magma, which gives it a very porous structure and makes it very light, with a water retention capacity far superior to that of volcanic gravel but still very good drainage, has a low nutrient retention capacity.
Very good for use as a drain in Bonsai soil mixes, as a bottom of the pot layer to ensure drainage.
It is used in higher percentages, in pre-Bonsai and plants in which you want great drainage to increase watering (and as such greater possibility of growth fertilization) and in Yamadori after collection for its good balance of retention X drainage of water, its mineral nature makes it poor in nutrient retention so it must be combined with a % of organic matter (eg Coco Peat).