Optical extinction due to surface plasmon resonances of spherical and polyhedral carbon onions prepared from diamond nanoparticles has been studied. Extinction spectra for the onions dispersed in the distilled water were obtained experimentally by optical transmission spectroscopy. Theoretical considerations were given to interpret the experimental results. For spherical onions, a defective spherical onion model was introduced and the experimental extinction features were traced back to the aggregate of the defective spherical onions. The extinction spectrum with two peaks for the aggregate of polyhedral onions was reproduced by treating the planar segments of the onions as ellipsoidal graphite nanocrystals with crystalline anisotropy. The present study may allow us to address a long-standing important problem about an origin of the interstellar extinction bump.