Here's why:

* Ultrabasic rocks are igneous rocks with very low silica content and a high content of magnesium and iron.

* Kimberlite is a specific type of ultrabasic rock that is characterized by its unique mineral composition and its formation deep within the Earth's mantle.

* Diamonds are formed under extreme pressure and heat deep within the Earth's mantle, and kimberlite pipes are the only known source of diamonds on Earth.

While other ultrabasic rocks exist, only kimberlite has the specific conditions needed for diamond formation and transport to the surface.