Großer Inselsberg is a rhyolitic butte that has withstood the weathering of the surrounding softer rock layers. The near-surface rocks of the summit region belong to the lower vulcanites of the Oberhof sequence in the lower Rotliegend and crop out in Reitsteine, a steep escarpment southeast of the summit. They are embedded in the conglomerates, sandstones and siltstones of the Goldlauter sequence to the north and the grainy gneiss of Cambrian origin to the south. An abrupt transition between the Inselsberg rhyolite and the gneiss can be observed along the west-east Inselsberg fault.