Stone Mountain is a granite dome, and granite is formed from magma that cools and crystallizes. It's difficult to give an exact age for the formation of Stone Mountain because:

* Igneous rock formation is a gradual process. The magma that cooled to form Stone Mountain didn't solidify overnight. It would have taken millions of years to cool and crystallize.

* Erosion has removed layers. Since the mountain formed, erosion has worn away some of its surface, making it difficult to determine the exact age of the original granite.

However, geological evidence suggests that Stone Mountain formed around 340 million years ago during the Carboniferous Period. This means it is a very old geological formation.