I have flagstone. I absolutely love how it looks. But flagstone is sandstone, and based on my use case, not particularly suited as a border for a pool. Sandstone is sand compressed into "rock." Done in layers over time, I think. And water is not kind to layers of sand. Flagstone tends to delaminate and either flake or revert back to sand. Mine is coming apart. I expect eventually replacing some or all of the stones will be needed. I've read others have had success with their flagstone. And claims have been made that the source makes a big difference, as in: flagstone harvested from one quarry works better than that from another. And that makes perfect sense to me.
So if you decide on a sandstone product, you should look into the track record of that specific product. Somehow find out where it comes from and how its cousins are doing in others' pools. Otherwise, there's a risk that yours won't perform as expected.
Only my flagstone coping is compromised. I have a ton of it elsewhere that is perfectly fine. The worst stones are those closest to the pool steps. Proof to me that it's the constant exposure to water that is causing the problem.
