Diorite is a coarse-grained intrusive igneous rock. It contains large interlocking, randomly oriented crystals. It is a dark coloured rock, usually medium to dark grey, containing many mafic crystals.

Mostly it looks like dark coloured granite.

Diorite Copyrighted image Credit: The Open University

How was it formed?
Diorite formed deep within the Earth’s crust from cooling magma that never made it to the surface. It usually occurs as quite small intrusions often associated with larger intrusions like granite. Slow cooling produces the large crystals.

Diorite forms from magma that does not contain a lot of quartz (silica) or the light coloured minerals that make up granite.

 

Get closer to geology