There isn't a single, universally accepted term for very smooth rocks. The most common term is "polished rock," which describes a rock that has been smoothed and sometimes even made shiny by natural forces like water, wind, or glacial movement.

Here are some other terms that are sometimes used:

* Cobble: A rounded, smooth rock, typically larger than a pebble.

* Pebble: A small, smooth rock, usually found in riverbeds or on beaches.

* Tumbled stone: Rocks that have been intentionally tumbled in a machine with abrasive materials to create a smooth finish.

* Glacial erratic: A rock that was transported by a glacier and dropped in a new location, often with a smooth, rounded shape.

The specific term you use might depend on the context and the size of the rock.