Here's why:

* Kimberlite is a type of igneous rock that forms deep within the Earth's mantle. It's characterized by its unique composition and its ability to carry diamonds to the surface.

* Pipes refer to the vertical, cylindrical formations created when kimberlite magma erupts to the surface. These pipes often have a distinctive carrot-shaped profile.

* Primary deposits refer to the original source of the diamonds, where they formed within the Earth's mantle. This is in contrast to secondary deposits, which are formed from erosion and transportation of diamonds from primary sources.

Therefore, kimberlite pipes are the primary source of diamonds mined on Earth's surface.