Diamonds form in the upper mantle of the Earth, specifically in a zone called the lithospheric mantle, which is about 100 to 200 kilometers (62 to 124 miles) below the surface.

Here's a breakdown:

* Upper mantle: This layer is situated above the lower mantle and below the Earth's crust. It's mostly solid rock, but with some molten material.

* Lithospheric mantle: This is the rigid, outermost part of the upper mantle. It's where diamonds form under immense pressure and heat.

Diamonds are formed from carbon under extreme pressure and heat over millions of years. The conditions necessary for diamond formation are only found deep within the Earth's mantle.