Forget what your seventh grade science teacher might have told you: scientists concur that most diamonds do not actually come from pressurized coal.
Perhaps the misinformation stems from both diamonds and coal being made of carbon. But the probability of diamonds forming from coal is quite low for a number of reasons.
Diamonds are thought to form deep in the mantle from a pure form of carbon called graphite. They are brought to the surface through volcanic eruptions, traveling through protective pipes made of an igneous rock called Kimberlite.
Coal, however, is a very impure mixture of organic material forms at a much shallower level in the Earth's surface. Technically, pressurized coal could lead to a diamond, but it would be a very impure one. Additionally, diamonds are also believed to be much older than Earth's earliest plants, from which coal is formed.
The tricky part is that these are more or less theories – it is hard to study diamond formation because of the great depth at which they form. Diamonds might come from a number of natural phenomena.
Geology.com's Dr. Harold King writes that the impact of an asteroid crashing into Earth can deliver the high temperature and pressure needed to form a diamond. In fact, diamonds have been discovered near impact sites. Researchers have also discovered diamonds in meteorites that have come to Earth. One scientific belief holds that diamonds were burrowed deep in the mantle through subduction zones, and brought to the surface from the force of volcanic eruptions through the Kimberlite pipes.