"River stone" is not a specific rock type, but rather a general term used to describe smooth, rounded stones that have been eroded and polished by water in rivers.

These stones can be made from various rock types, including:

* Igneous rocks: Like granite, basalt, and obsidian.

* Sedimentary rocks: Like limestone, sandstone, and shale.

* Metamorphic rocks: Like marble, slate, and gneiss.

The specific rock type of a river stone depends on the geological makeup of the area where the river originates and flows.

So, when someone refers to "river stones," you'd need further information to determine their exact composition.