Real stone is fine (at least most are). Just be wise in choosing--i.e. don't use any stones that would discolor water (this would mean its releasing something), don't use stones with extremely sharp edges/points, etc. I've used everything from lava rock to water smoothed stones. I don't know if I would use sandstone, as tyflier uses, because if it is anything like the sandstone in N.C., a lot of sand rubs off.
Whether it's store bought or found outside, I suggest always sanitizing first (even with fake plants and hides).
If you want to make a more natural looking store bought hide, you can always glue (using silicon or other 'safe after dried' adhesives) stones, bark, or whatever to the plastic hide.