Stone Mines in Age of Empires are fairly common and as such, acquiring stone is not usually a problem. Stone is used to build Towers and Walls.
Age of Empires II
Stone Mines in Age of Empires II are about as common as in Age of Empires, although more stone is used; as such, the mines run out more quickly, leaving trade as the only available method of acquiring more. Stone is used for building Towers, Walls, Castles, Town Centers, and Wonders, and for researching some technologies.
Poles have the passive ability to generate gold when mining stone.