Fluorescence Spectroscopy. The 3D fluorescence spectra for most of the N-V center samples are complicated by multiple fluorescence-producing defects, sometimes as many as three. Both naturally colored samples (21 and 22; figure 10A) showed the fluorescence emission feature of the (N-V)0 center, with ZPL at 575 nm and an emission band extending from 575 to more than 700 nm with peaks at 586, 600, and 618 nm. We also saw clear fluorescence from two defects: N3 (triplet peaks at 410–450 nm) and H3 (a 520 nm peak that maximized at 470 nm excitation). Despite its presence in PL spectra, fluorescence from (N-V)– was not observed in these samples. Sample 21 also displayed fluorescence of unknown origin at ~340–360 nm. The treated samples (23–25) contained both the (N-V)0 and (N-V)– centers as overlapping bands, with the latter fluorescence band dominating at 660 nm and extending beyond 750 nm (figures 10B and 10C). Sample 25 (figure 10C) featured only N-V defects in the fluorescence spectrum, consistent with the occurrence of mainly isolated nitrogen impurities (type Ib). Samples 23 and 24 contain aggregated nitrogen impurities and, as expected, showed fluorescence from N3 and H3 color centers.