Coal has rarely played a role in the formation of diamonds. In fact, most diamonds that have been dated are much older than Earth's first land plants - the source material of coal! That alone should be enough evidence to shut down the idea that Earth's diamond deposits were formed from coal.
Subsequently, one may also ask, can you make a diamond out of coal?
Diamonds and coal are both, at their base, different forms of the element carbon (C on the periodic table). And yes, pressure is a key part of what turns decaying carbon-based life forms such as plants into coal, as well as what turns carbon into diamonds. Coal is also mostly carbon, but it is hardly pure.
Can you manufacture a diamond?
A synthetic diamond (also known as an artificial diamond, cultured diamond, or cultivated diamond) is diamond produced in an artificial process, as opposed to natural diamonds, which are created by geological processes.
Can you make a diamond out of graphite?
One way to turn graphite into diamond is by applying pressure. However, since graphite is the most stable form of carbon under normal conditions, it takes approximately 150,000 times the atmospheric pressure at the Earth's surface to do so. Now, an alternative way that works on the nanoscale is within grasp.