Pure 3/8” horticultural pumice.
Quarried from southwest New Mexico- dug, hauled and packed by yours truly. Pumice is the absolute best aeration medium for arid plant soil composition. Far superior to perlite, turface or vermiculite for a plethora of reasons- namely acute moisture retention, optimal porosity, weight to mass ratio, mineral release and soil enrichment, pH neutrality, etc.
This should be mixed into all cactus and succulent soils as well as any plant needing controllable soil drainage. Some plants actually prefer to be potted in pumice alone!