Most of the ones I used were flint.
To attach the stones I used a hot glue gun which seemed to give quite a solid attachment then added the polyfiller directly, though I have also done it after a coat of PVA (white glue/Elmers) to give a rougher surface that other things can attach to.
Does it help if you use an old Stones' CD?
Probably, at least it will gather no moss.