They are either concretions or water-worn pebbles - either can be surprisingly regular in their ovalness.
If the veining seems to be symmetrical within the stones, concretion is more likely.
If the stones have concentric layers, concretion is more likely (the stone on the right of your first photo seems to show this).
If the veining seems to be independent of the surface of the stones (crossing randomly across them), pebble is more likely.
If you saw or break one in half, you may get a better idea of the structure of the veining. If the veins are widest near the middle of the stone, that strongly suggests concretion. Concretions with such veining are called septarian nodules (because they are divided into parts by the veins; septum is a divider).
(Re sawing or breaking: you'll see much more if you saw, then wet the surface.)
Although there is a supposed distinction between concretions and nodules, the terms are often used loosely and interchangeably. I won't go into that here.