how and where do diamonds form
1 Diamonds are formed deep within the Earth: between 100 km and 200 km below the surface. 2  The temperatures are about 900 - 1300 C in the part of the Earth's mantle where diamonds form. 3  The pressure is between 45 - 60 kilobars.
Unlike other gems which are formed by a combination of elements, diamonds are made up of one single element-Carbon. Carbon-containing minerals present in the Earth at those depths, crystallize to form diamonds because of the immense pressure together with the heat from molten magma.