Though diorite may occur independently, it is often associated with granite and gabbro intrusions, with which it sometimes merges. The rock is generally produced in the continental crust above subduction zones and is found in some forms worldwide. However, orbicular diorite is an extremely rare variety of the rock that contains many spherical lumps, or orbs, that exhibit concentric zones of light and dark color. Once only known on the island of Corsica, occurrences of orbicular diorite have now been found in South Africa, North Carolina, and a few other locales as well. Yet, how this unusual type of diorite forms has yet to be determined with certainty and is a matter of significant debate among geologists.