Relative classification of carbon allotropes by crystallinity and bond configuration, and the resultant effects on reflectance and UV upturn. The horizontal axis indicates the ratio of sp² (graphitic) to sp³ (diamond-like) bond configuration. The vertical axis indicates the degree of structural order. Color gradients symbolize relative reflectance and UV/visible reflectance effects, such that reflectance increases toward the upper right (shaded black to bluish-white), and spectral slope increases toward the lower right (shaded violet to brown). The UV upturn peaks at top left. Polycrystalline, amorphous, or glassy phases, which are ambiguously defined, fall below the dashed arc.