Crushed Pebbles
Crushed pebbles are sold in the UK by a company named Chempak who offer all kinds of minerals, grits, etc. These pebbles are very rough on the surfaces and sharp on the edges and impervious to water. You also can used crushed pumice or brick to same effect. Cacti roots love this material and when you eventually transplant the seedlings, this material will be quite firmly attached to their roots.
