We compute the optical properties of spherical dust grains composed of hollow silicate cores coated with two carbonaceous layers, the inner layer of mostly graphitic sp2 carbon and the outer layer of sp3 polymeric amorphous carbon, and explore the consequences on modelled interstellar extinction curves. A family of curves is created by varying the thicknesses of the sp2 and the sp3 layers. The average galactic extinction curve is well matched by one of this family of computed curves.