[Insane Reefer]

Sure you can Custom, but many folks have a hangup about silica sands. If you use calcium based sand, your rock has a higher over-all calcium content, which may be better as the more Calcium Carbonate in the rock, the more like RLR it becomes. There is also a vague potential for "Alkali-Silica Reaction" when using silica...

Personally, I use Calcium Carbonate from the feed store, but I know that several people here have used silica sands - both blasting sand and playsand, to make rock, and they don't seem to notice a real difference, so, IMO, it comes down to preference...