The weight of the rocks above the mantle results in "high pressure". Diamonds need high "temperatures and pressure" to form from "carbon". Diamonds form in the mantle in certain zones were conditions are right for them to do so. There just happens to be the right critical "Tempature, pressure, mineral" environment in the mantle in certain zones for their formation.
the element that is formed under extreme "heat and pressure" is "carbon"
The "pressure" mantle is 23g/cookies 5.
I hope this helps.