Diamonds that form at about 10 GPa (10 gigapascals) form in the mantle layer of the Earth.

Where 10 GPa occurs in Earth

  • Typical pressures in the upper mantle at depths where most natural diamonds grow are on the order of 5–6 GPa at ~150–200 km, increasing with depth into the mantle.
  • A pressure of about 10 GPa corresponds to greater depths still within the mantle, not the crust or core, so such diamonds are classified as mantle-formed.

Why not crust or core?

  • The crust is too shallow and low-pressure; it never reaches anywhere near 10 GPa, so diamond stability from carbon there is not natural under normal geologic conditions.
  • The core has vastly higher pressures than 10 GPa and very different composition (iron‑rich metal), while gem diamonds sampled at Earth’s surface are known to have originated mainly from 150–250 km depth in the mantle.

Extra context: diamond-forming zone

  • Most natural diamonds form in a region roughly 140–250 km deep, firmly inside the mantle, and are later brought to the surface by fast-rising volcanic rocks called kimberlites.
  • Some “super-deep” diamonds form even deeper in the lower mantle at pressures well above 10 GPa, but they are still mantle diamonds, just from greater depth.

So for a pressure of about 10 GPa, the correct Earth layer is the mantle.