The two main stone circles of the Ōyu Stone Circles are made up of approximately 8,500 riverbed stones, of which around 65% comprise so-called quartz diorite porphyrite. It is thought that these stones were transported from Ōyu River, located about 2–4 km from the site. The quartz diorite porphyrite comes from Mt. Morosuke in the upper reaches of Akuya River, a tributary of Ōyu River. The mountain is a columnar jointing characterized by the same beautiful green as jade, and it is thought that the rock flaked away and the pieces became rounded as they traveled downstream where those of appropriate size were selected.