Diamonds are found deep within the Earth's mantle, specifically in a region called the lithosphere.

Here's a breakdown:

* Mantle: The layer of the Earth below the crust, extending to about 1800 miles deep.

* Lithosphere: The rigid outermost layer of the Earth, composed of the crust and the uppermost part of the mantle.

Diamonds are formed under immense pressure and heat deep within the lithosphere, usually between 90 and 150 miles down. They are brought to the surface through volcanic eruptions called kimberlite pipes. These pipes are like giant vertical tubes that carry the diamonds and other minerals upwards.

While most diamonds are found in kimberlite pipes, they can also be found in other types of volcanic rocks, like lamproite.

Here's a simplified visual:

```

Earth's Layers:

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Atmosphere

Crust

Mantle (Lithosphere) -> Diamonds formed here

Outer Core

Inner Core

```

So, the next time you see a sparkly diamond, remember that it has journeyed from the depths of the Earth!