Dunite granular, green igneous rock composed of coarse grains of olivine, is the source of the world's supply of chromium. It weathers to a dun brown color, as at Mount Dun, in New Zealand, its namesake. Some dunites are rich in chromite, magnetite, ilmenite, or pyrrhotite; they may also contain a small amount of pyroxene. Dunite occurs in layered, gabbroic igneous complexes (see gabbro). It probably forms from the accumulation of dense, early crystallizing grains of olivine that sink to the bottom of low silica magma. Intrusions of dunite form sills or dikes. Some dunite has been altered to form serpentine.