The majority of Schiehallion consists of quartzite, a rock formed from an original sand by heat and pressure. Quartzite is a white or pinkish rock, in which the dominant mineral is quartz. People often mistakenly think that Schiehallion is an extinct volcano because its shape is reminiscent of an active volcano; in fact, it was formed by sedimentary processes and the shape is the result of erosion by ice during the ice ages.