[CH]/[CH₂] ratios for ‘spherical’ nano-diamonds with actual radii ≤10 nm (black line), showing the inherent dispersion in the data. The red line shows the analytical approximation to these data (see text for details). The grey shaded gives an indication of the observed and experimental values (0.5–3) and the lighter grey verticalband indicates the typical radii of the most abundant pre-solar nano-diamonds (1.3–1.5 nm).