Orbicular diorite rock. Diorite is an igneous rock that forms when volcanic magma solidifies under pressure in the Earth's crust. This variety of diorite is called orbicular diorite, napoleonite or corsite, after the island of Corsica where it is commonly found. It contains spherical lumps made up of concentric layers, embedded within a regular diorite matrix. This polished sample shows such spheres, which range in diameter from 40 to 90 millimetres.