Flagstone causing rust in pool

I am a pool service provider. I have a pool surrounded by flagstone. All the coping is flagstone. There are definite dark markings (rust) on most of the stones . The pool is around 4 years old. The customer has small dot rust stains beginning to form on flat spots on his pool. I can see the natural deterioration (dirt) from the rocks.We ruled out fertilizer construction work rusty patio furniture etc.. Is there enough iron or metal in these flagstone pavers to leave small rust spots in his pool?
Thank you for this forum.
Scott