what is the process in which diamonds form?

Answer 1

Diamonds can form under relatively low pressures and low temperatures, but they will not be large, gem quality diamonds. The diamonds from southern Africa were formed under igneous conditions in the upper mantle and brought to the surface through kimberlite pipes. This occurred a few billion years ago and the reasons for their formation are not well understood.

Answer 2

The element carbon, when exposed to high heat and pressure, will form diamonds. It is still carbon, but in a different solid form.