Making Diamonds page 77

The highest pressures on 

the Earth are found under its

rocky surface which is called crust. Earth's crust can be more than ten miles sixteen km thick in place. The heavy rock creates a huge pressure. This pressure forms many kinds of rocks such as slate and sandstone within the crust. Natural diamonds are formed even deeper  down the Earth's mantles at about ninety miles one hundred fifty km below the surface where temperature are blazing hot at least twenty hundred degrees ten hundred fifty degrees. When the hot rocks which contain carbon and other minerals are squeezed by the Earth's pressure which is around six million times more than air pressure beautiful, superhard dimands can form.