Coal can turn into diamond after high heat and pressure. It first turns into graphite and eventually into diamond. However, there are much more extreme conditions required when working with less pure substances. For more information, check out http://www.ehow.com/facts_7318475_coal-turns-diamonds.html.
Petroleum. Otherwise, nothing. It is a myth that coal turns into diamonds. Diamonds are a carbon crystal, and there are man made machines that can turn minerals such as graphite with a high carbon concentration into machine-grade diamonds, but coal does not become diamonds.