Feature Article
A Comparison of
R-line Photoluminescence of Emeralds
from Different Origins
D. Brian Thompson, Joshua D. Kidd, Mikko Åström,
Alberto Scarani and Christopher P. Smith
A fundamental task for gemmologists is determining whether an emerald is
natural or synthetic. Within the laser-excited photoluminescence spectrum of
emerald, the peak positions and relative intensities of two emissions in the
680–685 nm range, known as R lines, can help identify if a sample is natural,
and can also provide information about its geological origin. In particular,
the R1 line of synthetic emerald is positioned at the shortest wavelength,
while for natural emeralds with a non-schist origin this line is found at the
same or longer wavelengths, and for schist-type emeralds the line peaks
at an even longer wavelength. This measurement can supplement origin
results obtained from established methods, such as inclusion microscopy,
spectroscopy (e.g. ultraviolet-visible–near infrared [UV-Vis-NIR], Fouriertransform infrared [FTIR] and Raman) and trace-element analysis by laser
ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS).
The Journal of Gemmology, 34(4), 2014, pp. 334–343, http://dx.doi.org/10.15506/JoG.2014.34.4.334
© 2014 The Gemmological Association of Great Britain
Introduction
Emerald, the green to bluish green variety of
beryl [Be3Al2(SiO3)6], is a beautiful and important
gemstone (e.g. Figure 1). Pure beryl does not
absorb visible light and therefore appears
colourless. The green colour of emerald results
from trace amounts of Cr and/or V; Fe may add a
yellowish or bluish tinge. A Cr3+ ion substituting
for Al3+ at its crystal site is surrounded by an
octahedral arrangement of six oxygen ions. Then
six silicon and three beryllium ions, the nextnearest neighbours with which the chromium
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shares the oxygens, result in a trigonal distortion
of this crystal site (Wood, 1965). Crystal field
theory explains how the three electrons in a Cr3+
ion’s d orbital, when placed in beryl’s octahedral
oxygen field, absorb light across the red-orange
and blue-violet wavelength ranges, so that
emerald only transmits light in the green range
(Wood et al., 1963; Wood, 1965; Mitra, 1996;
Avram and Brik, 2013).
Chromium is the only emerald chromophore
that exhibits photoluminescence (PL); vanadium
does not cause PL in emerald. According to
The Journal of Gemmology, 34(4), 2014