DISCUSSION
The vast majority of the fluorescence spectra for the
natural-color diamonds tested fell into three categories
with respect to the peak wavelengths and
shapes. In fact, only two of the 62 natural-color diamonds
described in this article (again, see figure 11)
had peak positions other than those described in the
three major categories (~450 and ~490 nm; ~525
nm; and ~550 nm). As shown in figure 13, the colored
diamonds we tested are largely segregated into
these three categories on the basis of their bodycolor.
The information conveyed by these categories
may provide clues to the origin of their color.
Category 1: Fluorescence Spectra with Dominant
Peaks at ~450 and ~490 nm. For all the diamonds
represented in figure 4, the fluorescence spectra
show double peaks with maxima at ~450 (±12) nm
and ~490 (±13) nm. This spectral pattern generally
manifested itself as blue and is likely representative
of fluorescence spectra for the vast majority of bluefluorescing
diamonds (e.g., Moses et al., 1997).
UV-Vis-NIR spectra of three natural and three
irradiated diamonds showed the presence of the N3
center—a grouping of three nitrogen atoms in a
(111) plane surrounding a vacancy—and higher-resolution
spectra using the spectrofluorometer all
confirmed that the ~450 and ~490 nm fluorescence
peaks in the blue region of the spectrum are derived
from the N3 center. The band centered at ~490 nm
is artificially high in intensity, which is an artifact
of the CCD spectrometer. On a spectrum for one
sample obtained with the CCD spectrometer,
radiometric correction of the output intensity
across the wavelength range decreased the intensity
of the ~490 nm peak so that it conformed better
with spectra obtained from higher-resolution spectrofluorometers,
thus confirming that the two