How diamonds form
Carbon on its own cannot form into diamonds even deep below the surface of the Earth. There are a few things that have to happen in just the right way. First of all the subsurface carbon is located about 150 km below the Earth's crust has to be subjected to immense heat and pressure. Coal and diamonds are both composed primarily of carbon. Their chemical structures have marked differences. Coal is formed from highly impure carbon. This carbon generally contains the following elements: oxygen, selenium, hydrogen, nitrogen and sulfur. Diamonds on the other hand require an absolutely pure source of carbon. If there is any defect in purity the diamond will generally change color or become too included to be worked by diamond jewelers. When carbon sources which are very pure such as carbon dioxide are trapped deep under the Earth’s surface, conditions are ideal for the formation of diamonds. 725,000 PSI is about the level of pressure needed to compress the carbon into a diamond