Sixty-seven natural blue diamonds, including the two largest such gemstones known (the Hope and the Blue Heart), were probed by ultraviolet radiation, and their luminescence was analyzed using a novel spectrometer system. Prior to this study, the fiery red phosphorescence of the Hope Diamond was regarded as quite rare compared to greenish-blue phosphorescence. However, our results demonstrated that virtually all blue diamonds phosphoresce at 660 nm (orange-red) but that this emission often is obscured by a concomitant luminescence at 500 nm (green-blue). Although both bands were nearly always present, the relative intensities of these emissions and their decay kinetics varied dramatically. Consequently, phosphorescence analysis provides a method to discriminate among individual blue diamonds. Treated and synthetic blue diamonds showed behavior distinct from natural stones. Temperature-dependent phosphorescence revealed that the 660 nm emission has an activation energy of 0.4 eV, close to the 0.37 eV acceptor energy for boron, suggesting that the phosphorescence is caused by donor-acceptor pair recombination.