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 (Δ_fH^θ) 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 (Δ_fH^θ ≈ 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.