Things to consider with RoR.
End state / size of tree: Your tree will grow, your rock won't. Often folks use to small a rock and the tree "out grows" it.
Size of rock: see above.
Color: Does the rock pair well or contrast with the tree.
Texture: does the rock reflect age, ruggedness and the geology where your tree would be located in. Smooth, rough, will roots attach.
Shape: will it compliment the tree or overpower it.
Material: will it last as long as your tree.
Will it crumble after years of watering, sun, rain.
Will exposure to winter freezes and thawing cycles cause it to split or frag.
Once its in place it is pretty much committed for the duration.
I have seen quite a few RoRs where all you are left with is some messed up exposed roots because attention was not given to the "future" of the material.
Good luck.
