If you pass construction sand (for making concrete) through a window screen and lose the fines, you will end up with something that is essentially poultry grit (the granite kind). Around here that's the cheapest thing available for making a rocky mix. The key is to avoid any sand that has other stuff added (like wax or whatever) as there are treated products out there as well, for specialty purposes.
Pictured: pumice, grit from sand, lava rock (all suitable for this purpose).
