Coal forms when organic matter, mainly plants get buried to depth on the order of kilometers where the pressure can be calculated from pressure= density of overlying rock x gravity x depth.

So a depth of ten km in Earth is pressure of ~ 0.3 GPa.

At any rate due to pressure and T at depth, the plant material loses a lot of its H and O and because the oxygen pressure is so low, this plant material gets reduced to elemental carbon.

Different types of coal (lignite, bituminous, anthracite) form mainly due to the temp t which they are held.... we are taking about 100's of degrees C , roughly...

Recall that on Earth the temperature increases by about 25 deg C for every kilometer down... so at 10 km typical T is 200-300 deg C.

Without plants there would be no coal.

Coal forms from compressed plant material, and there are many different grades of coal formation, ranging from peat (which forms at or just below the surface), to graphite, a mineral that is basically just a form of carbon. Anthracite is the hardest and densest form that is normally referred to as "coal".

I believe coal can remain stable for temperatures up to about 200 degrees C, above which it tends to change into other materials (including graphite). Coal is common in the certain parts of the Appalachians mountains, though, which means it can survive the pressures and temperatures of being buried kilometers beneath the surface.

I must admit to being surprised at how difficult it is to find information looking for this question. I wonder how much is not even known...