We have used different spectroscopic techniques to obtain a consistent quantitative absorption spectrum of a sample of meteoritic nano-diamonds in the wavelength range from the vacuum ultraviolet (0.12um) to the far infrared (100um). The nano-diamonds have been isolated by a chemical treatment from the Allende meteorite (Braatz et al. 2000, Meteorit. Planet. Sci., 35, 75). Electron energy loss spectroscopy (EELS) extends the optical measurements to higher energies and allows the derivation of the optical constants (n & k) by Kramers-Kronig analysis.