What is this kind of rock called?

Question

It is common along creeks and rivers in the midwest. It has a brown/tan outside but when you break it open, it is a grey/bluish color. The rock is smooth on the outside and is smooth on the inside too. its not a grainy stone.

Answer 1

Only the interior of the rock is important.  The exterior has been modified by erosion and chemical actions.  Without seeing it I can only guess, but I think it might be basalt.

Answer 2

There are several it could be from that description. Limestone, shale, and sandstone are very common, but are usually grainy. Chert is also very common and is not grainy. Chert can come in many colors. It's a low grade variety of flint and is the rock Navive Americans often made arrowheads out of.

See if you can easily scratch it with a nail. Limestone and shale will scratch easily, but chert is much harder.

Igneous rocks like basalt are not very common in the Midwest, but locally (like around Iron Mountain in the Ozarks) it can be abundant.

It's a pretty wild guess, but I think chert is most likely.