Coal is carbon mixed with minerals.
Being a solid, it retains physical features that divulge its origin. The existence of fossil fern leaves in coal tells us that coal came from prehistoric plant life. If that weren t enough of a clue, the world has many peat beds where decaying vegetable matter, in the absence of oxygen is found in various stages that will ultimately lead to coal. Oil, however, is not so easily understood. It is all too easy to say, without providing specifics, that sometimes geologic heat and pressure lead to coal, and sometimes they lead to oil.