coal is one of the important fossil fuels and it takes millions of years to be formed. This is also an exhaustible natural resource, so we have to use it judiciously so that it can be available for our future generations as well. If we go on consuming it at a faster rate then it might get exhausted very soon as the rate at which it is formed is very slow.
When layers are compacted and heated over time, deposits are turned into coal. Coal is quite abundant compared to the other two fossil fuels. Iceland is the country least dependent on fossil fuels in the world.