This article presents the latest results of GIA's research on the interaction of light with fully faceted colorless symmetrical round brilliant cut diamonds of various proportions. The second major article in this three-dimensional modeling study, it deals with fire—the visible extent of light dispersed into spectral colors. As fire is best seen with directed (spot) lighting, the metric for fire presented (dispersed colored light return, or DCLR) uses this lighting condition. DCLR values were computed for more than 26,000 combinations of round brilliant proportions. In general, different sets of proportions maximize DCLR and WLR (weighted light return, our metric for brilliance), but there are some proportion combinations that produce above-average values of both metrics. Analysis of these metric values with variations of five proportion parameters demonstrated that every facet contributes to the appearance of a round brilliant diamond. In particular, star and lower-girdle facet lengths—which are ignored by most cut-evaluation systems—could have a noticeable effect on WLR and DCLR. Observations of actual diamonds corroborate these results.