Here's a breakdown:

* Granite: Stone Mountain is composed primarily of granite, a hard, igneous rock formed from slowly cooled magma deep within the Earth's crust.

* Dome: The granite body has a distinct dome shape, created by the upward pressure of the magma as it intruded into surrounding rock layers.

* Laccolith: While technically a dome, the large size and specific formation of Stone Mountain qualify it as a laccolith. Laccoliths are large, dome-shaped intrusions of magma that push up overlying rock layers.

The granite of Stone Mountain was formed about 300 million years ago during the Paleozoic Era. Over time, erosion stripped away surrounding rock layers, exposing the massive granite dome we see today.