The most stable form of carbon is

Carbon exists in several allotropic forms, including diamond, graphite, and fullerenes [2]. While diamond is the hardest known substance due to its rigid three-dimensional covalent structure, it is actually metastable at standard temperature and pressure [3]. Thermodynamic data and phase diagrams reveal that graphite is the most stable allotrope of carbon under ambient conditions. In graphite, carbon atoms are arranged in hexagonal layers with sp2 hybridization, which is energetically more favorable than the sp3 arrangement in diamond at normal pressures [2]. Although diamond is kinetically stable because the activation energy for its conversion to graphite is extremely high, graphite remains the thermodynamically preferred state [3]. Other forms like fullerenes and coal are less stable; coal is an impure form of carbon, and fullerenes are molecular allotropes that do not reach the thermodynamic stability of the graphite lattice [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Allotropes of carbon > p. 61
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > Table 3.1 > p. 40
3. [3] https://www.sciencedirect.com/topics/chemical-engineering/carbon-structure