Studies of diamond nucleation and growth on conical carbon tubular structures show that the nucleation preferentially occurs at the tips, but only occurs on the sidewalls when they are pretreated with diamond or other powder dispersions, forming a nanodiamond coating. The high-resolution transmission electron microscopy (HRTEM) studies reveal that the diamond nucleation on the sidewalls may proceed through the formation of diamond nuclei within the walls at subsurface damage sites caused during pretreatment. In the case of experiments with low atomic hydrogen conditions, carbon onion structures are observed on the sidewalls but only with pretreatments.