To turn carbon into diamonds, it takes between 237,000 times atmospheric pressure (24 gigapascals) to 1.3 million times atmospheric pressure (136 gigapascals). High pressure and high temperatures in the earth's lower mantle crust are pertinent for the creation of diamonds. However, diamonds are made from carbon; not coal.
Diamonds are formed when carbon is under an immense amount of pressure and heat. The carbon atoms found in coal are squeezed and heated, and then pushed toward the earth's surface, where they cool and become diamonds.
Plants are the source material for coal, and most diamonds are much older than the earth's first land plants. This means the earth's diamond deposits were created long before the existence of coal.