Composed of of basaltic andesite and of andesite, Diamond Peak is a shield volcano, though it might be considered a modest stratocone because of its steep slopes and the pyroclastic materials at its core. Having produced andesite, Diamond Peak represents one of the few mafic shield volcanoes in the Cascades that are known to grade into andesite, along with Mount Defiance, Mount Bailey, Devils Peak, and Prospect Peak. It consists of a main cone made of pyroclastic material including palagonitized rock along with basaltic andesite cinders and glassy scoria, as well as thin layers of basaltic andesite lava flows. Diamond Peak has two overlapping volcanic cones, with the elder edifice making up its lower northern peak. The volcano does not show evidence of producing lahars.