A Stone Mine.
Stone is used to create Walls, Towers, and other Units or Buildings. Players can obtain stone by mining it from the ground. It appears as a gray dot on the mini-map. Each stone mine holds 300,000 units of stone, and allows up to 6 Citizens to collect stone at one time unless you have enabled Advanced Mining (then you can have 7). Stone isn't one of the major resources in Empire Earth, but it is needed in some cases.