Thermodynamically the most stable form of carbon is

Under standard conditions of temperature and pressure (298.15 K and 1 bar), graphite is the most thermodynamically stable allotrope of carbon. In thermochemistry, the standard enthalpy of formation (ΔfHθ) of graphite is taken as zero, serving as the reference state for the element carbon. While diamond is extremely hard and stable, it is technically metastable at room temperature and pressure; it has a higher Gibbs free energy than graphite (ΔfHθ ≈ 1.9 kJ/mol) and will spontaneously, albeit incredibly slowly, convert to graphite over geological time. Fullerenes (like C60) are less stable than both graphite and diamond. Coal is an impure, amorphous form of carbon and is not considered a standard pure allotrope. Graphite's stability is attributed to its sp2 hybridized carbon atoms arranged in hexagonal layers with delocalized π-electrons.