Here's why:

* Formation: Diamonds form deep within the Earth's mantle under immense pressure and heat.

* Transformation: They are created from carbon, which is a common element in the Earth's crust. This carbon undergoes a process called metamorphism, transforming from its original form into a crystalline structure.

* Not igneous or sedimentary: Diamonds are not formed by cooling magma (igneous rocks) or the accumulation of sediments (sedimentary rocks). They are formed through a process of transformation within existing rock.