A diamond is a crystalline mineral form of the element carbon.  Igneous, metamorphic, and sedimentary are descriptive terms used to  classify types of rocks, not minerals su … ch as diamond.     Diamond is accepted as a valid mineral by the International  Mineralogy Association. Sedimentary rocks are formed by the lithification of the sediments  which accumulate from the erosion of … preexisting rocks of all  types. They also included rocks form from animal and plant remains.   Metamorphic rocks are formed when sedimentary or igneous rocks are  heated and or squashed. Answered.