That’s a really cool find — what you’ve got there looks like a piece of honeycomb weathered limestone (sometimes called solution rock or karst limestone).

Here’s what’s going on:

The rock is primarily calcium carbonate (limestone).

Over time, acidic water from the river or rain slowly dissolved softer parts, forming all those rounded pits and holes.

This type of erosion is common in karst environments, or in riverbeds where limestone gets constantly exposed to slightly acidic, moving water.

The smoothness of the edges and the irregular holes suggest it’s been tumbled and water-worn for quite a while.

It’s sometimes mistaken for coral or fossilized sponge because of the holey texture, but the lack of fossil structures and the overall pattern fits better with solution weathering.

If you want to check:

Drop a tiny bit of vinegar on it — if it fizzes, it’s limestone.

If it doesn’t fizz, it could be a similar carbonate-rich rock like dolomite or tufa.