Geologically the Kellerwald, made up mainly of palaeozoic rocks, belongs to the Rhenish Slate Mountains. Scenically, however, it is also grouped as a separate entity with the Hessian Basin, because the rivers Eder and Itter form a natural boundary. Important rock types include the so-called Kellerwald quartzite, radiolarian rock, shale, greywacke and diabase. A regional peculiarity is the dark red Kellerwald agate, an Eisenkiesel, a quartz that is turned opaque red with hematite inclusions.