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 lattice with prefect 3-fold symmetry. this has to be coupled with high temperatures of about 1204˚C.
Only when these conditions are met can diamonds begin to form The carbon atoms begin to bond with four other carbon atoms. This creates the type of lattice which is the source of a diamond’s strength.
The diamond forming is only the first part of the process. All diamonds we get to access have been shuttled closer to the Earth's surface by geological movement.

How diamonds are brought to the surface

Diamonds are mined all over the world in areas that were brought to the surface through volcanic eruptions. Some of these eruptions are known to have been quite violent in nature. Scientists believe that the process of moving them from their formation location to the surface would have occurred very rapidly, in as little as a few hours. The force required to accomplish this would have been tremendous. It would have required volcanic eruptions traveling at roughly 30 miles per hour. The diamonds would then have been spewn out and strewn about the surface. The main reason scientists believe the process would have occurred with such speed is because if diamonds were subjected to the intense lava for extended periods of time, then they would have formed into graphite on the way up.
After volcanic eruptions brought the diamonds to the surface, they became contained in a material called Kimberlite. This is essentially cooled volcanic rock. When sealed inside of Kimberlite, they can maintain their natural hard and clear form. One aspect of diamond creation researchers struggle to understand is how long it takes for diamonds to form. Exact conditions of natural diamond formation are impossible to replicate in a lab, so the best determination method is carbon dating. Dating diamonds in this way give in an estimated age of between hundreds of millions to billions of years.