General:
* A pile of stones
* A heap of stones
* A cluster of stones
* A collection of stones
More specific:
* A cairn of stones (a pile of stones often used as a marker)
* A bed of stones (stones used in a garden or for a path)
* A scattering of stones (for a more random arrangement)
* A field of stones (for a large area covered in stones)
Ultimately, the best collective noun to use depends on the specific situation and the way you want to describe the stones.