I don't know about expanded shale, but I used pea gravel from the landscaping department at Lowe's. It was pretty cheap and seemed to serve its purpose well, just needed to be sifted and rinsed.

This post was edited by weirdflowers on Sun, Mar 23, 14 at 19:51

For grit it is the size that is important. What is the size of "expanded shale"?