It has been known that pentagons and heptagons in hexagonal graphitic network
give rise to a certain amount of curvature in the three dimensional structure
of graphitic carbon materials. The amount of curvature is quantized due to the
symmetry of graphite and, as a result, the structure formed by the network is
also restricted. We clarify the effects of curvature quantization on the forms
of graphitic carbon materials, employing the knowledge of differential
geometry, especially the Gauss-Bonnet theorem.