The short answer is diamonds are not found in coal mines. Diamonds do not occur in coal deposits. Coal will, under no condition convert into diamonds even though both are made up of carbon. read more
In short - not in coal mines, but in salt mines. Geological conditions required to store oil are impermeable rock capping above and below. Coal deposits are solid and do not require the same formation conditions. The most common mining method for underground coal mines is longwall mining. read more