Coal is a rock - an organic sedimentary rock. Unlike the rocks or stones that are often seen in our surroundings, it is composed mostly of organic (carbon containing) material, with a smaller
amount of inorganic minerals. Actually, the amount of organic material can vary quite a bit. Generally, anything with over 33% minerals is considered as shale, rather than coal.

Why does coal present such a tough scientific problem? A major reason is that it is derived from a complex mix of starting materials. The organic material that makes up coal comes from ancient
swamps. Go out to the nearest bog or marsh and grab a handful of the stuff that oozes around your feet. That's called peat, and it has the potential to become coal in millions of years. But the
plants you see around you are not the whole story, just one small part. For coal to form requires a whole bunch of events to happen at just the right time. We call the entire process coalification.

To start with, take a look at the black and white image of a piece of coal at the left. This is a from a piece about
two inches across; it is a typical chunk that you
are likely to find on any coal pile.

Notice the stratified dark and light bands. Coal is a sedimentary rock, and the bands represent the process of laying down different layers of organic material. In the case shown here, the dark bands actually are derived
from twigs, branches, roots, and other woody parts of the tree, while the lighter grey material also contains spores, resins, and charcoal  like materials. We discuss this in more detail in the coal maceral section. The dark bands shown here really are highly reflecting, but the way the light source was positioned to bring out the banding, they look black. The grey areas have a dull appearance. So even with the naked eye we can see the heterogeneous nature of coal.

Information provided by: http://chemistry.anl.gov