a good basic test is to put vinegar (acid) onto a rock, if it fizzes dont use it as its best to steer clear of limestones, sand is okay as long as you dont use builders sand.... the red stuff, play sand is probably your best bet and can be picked up quite cheaply at garden centres etc, also a good source for rocks. i work in a garden centre and get most of my rock from there, slate is my favourite as its inert, i believe, best way is to find broken off bits in the rocks and quite often they'll let you have those for nothing, providing they're not too large or too many at a time, you can also get good sized bags of gravel from these places,at lower prices than at an lfs, look for split bags and ask for a discount ...wink wink