Kostal Cone is made of fragmented and solidified lava called cinder and its summit contains a bowl-shaped crater. Kostal's cinders were ejected by lava fountain eruptions and accumulated around the volcano's vent in the shape of a cone when they fell back around its surroundings. Lava flows from Kostal's 400 BP eruption are basaltic in composition and forms a lava bed. This lava bed dams the southern end of McDougall Lake and is just one of the examples of volcanic activity that have occurred in the Wells Gray-Clearwater volcanic field since the last glacial period; others include the "Dragon's Tongue" lava flow from Dragon Cone just north of Kostal Cone.
There has been activity at this site as recently as 7,600 years ago, though more likely less than 1,000 years ago. Kostal Cone is too young for the commonly used potassium-argon dating technique (usable on specimens over 100,000 years old), and no charred organic material for radiocarbon dating has been found. However, the uneroded structure of the cone with the existence of trees on its flanks and summit have it an area for dendrochronology studies, which reveals the growth of tree-ring patterns. Tree-ring dating has revealed an age of 400 years for Kostal Cone, making it the youngest volcano in the Wells Gray-Clearwater volcanic field and one of the youngest volcanoes in Canada.