Quite a few minerals stick to themselves, so water seeping into a cavity that was eroded out might fill with one type of material over time. Basically the same process as fossilization.
Being round or saucer shaped would be consistent with a pool of water that lets the material bits group together over time, and as the surrounding materials start to cover the area it would curve on top as the exposed area is reduced.
That is one way, but there are a lot of other similar ones to. But in general some process that involves liquids moving materials into cavities or exposed puddles drying lets materials stick to themselves and form rocks.
Flowing water after they formed could wear off the rough edges as well over time.