Colloidal suspensions of nanometer quasi-spherical sized carbon dots and nanodiamonds have been prepared and their third-order nonlinear optical response was investigated by means of the Z-scan technique, employing 35 ps and 4 ns, 1064 and 532 nm laser excitation. Carbon dots were found to exhibit significant nonlinear optical response only under visible excitation for both pulse durations, due entirely to nonlinear refraction. On the other hand, nanodiamonds were found to exhibit significant nonlinear optical response at both excitation wavelengths under ns laser pulses, while in the case of ps excitation, they exhibited sizeable nonlinear optical response only for visible laser pulses. Nevertheless, carbon dots were found to exhibit significantly larger nonlinear optical response than that of nanodiamonds under all experimental conditions examined in the present study. Additionally, the optical limiting behavior of nanodiamonds was investigated in the ns regime, using both visible and infrared laser pulses. Nanodiamonds were found to exhibit important and broadband optical limiting efficiency making them possible candidates for photonic and/or optoelectronic applications.