Post on 21-Mar-2021
Verification of the Authenticity
and Classification of Gemstones
Using XRF
Introduction
For thousands of years, pearls, emeralds,
rubies and sapphires, as well as other precious
jewelry and mineral stones such as alexandrites,
aquamarines, etc., have been greatly valued due
to their rarity, finite nature and beauty. Nowadays
they are mainly used in the jewelry industry,
science and gemology.
A WHITE PAPER FROMSPECTRO ANALYTICAL INSTRUMENTS
It has even become technically possible to
artificially manufacture gemstones. These artificial
gemstones are usually of significantly lesser value
than natural gemstones, but they are scarcely
different in color and shape. In addition, natural
gemstones and pearls vary in value depending on
their origin due to their color, color properties when
exposed to light and their prevalence in nature.
When results matter
2 Verification of the Authenticity and Classification of Gemstones Using XRF
METHODOLOGYSo how is such a differentiation of gem-
stones, whether synthetic or real or whether
salt or freshwater pearl, possible? An ele-
mental analysis by means of energy disper-
sive X-ray fluorescence analysis (ED-XRF)
provides information on the elemental com-
position of gemstones and pearls and is also
completely non-destructive. The sample is
not damaged and can also be directly ana-
lyzed without having to prepare the sample.
The analysis can be performed with various
spectrometers, such as the SPECTROCUBE,
SPECTRO MIDEX or SPECTROSCOUT, the
latter of which can also be used on site, the
SPECTROCUBE and the SPECTRO MIDEX
can also be used for the analysis of precious
metal alloys. For the analysis of pure unpro-
cessed stones and pearls, devices in which
the samples to be examined can be placed
directly on the surface and analyzed from
below have proven to be advantageous.
For the analysis of jewelry and thus complex
geometries, analysis from above is more
advantageous as it is done by the SPECTRO
MIDEX. This device has an integrated video
system and supports the positioning and
documentation of the respective measuring
point and can also analyze the precious met-
al quickly and precisely.
Figure 1: The SPECTRO MIDEX offers an
integrated video system
3Verification of the Authenticity and Classification of Gemstones Using XRF
Such measurements allow to differentiate
between artificial and natural gemstones.
Figure 2 shows a comparison spectrum of an
artificial and a natural sapphire.
Artificial gemstones can be distinguished
from natural gemstones based on their col-
oring elements. In the above spectrum, the
blue color of the synthetic sapphire has been
created by adding cobalt oxide. However, no
cobalt oxide is present in the natural gem-
stone. Instead, the color originates from iron
oxide.
Figure 3 shows a comparison spectrum of a
natural and an artificial ruby. Traces of addi-
tional elements such as gallium and bismuth
are detectable in natural gemstones, which
are not present in synthetic gemstones.
These are spectrally purer, so they contain
hardly any additional trace elements.
Figure 2: Comparison spectrum of a synthetic (red spectrum) and a natural sapphire
(blue spectrum)
Figure 3: Comparison spectrum of a synthetic (red spectrum) and a natural ruby
(black spectrum)
Energy [keV]
Energy [keV]
Impu
lses
[x10
4 ]Im
puls
es [x
104 ]
4 Verification of the Authenticity and Classification of Gemstones Using XRF
It is also possible to differentiate based on
main components, i.e. whether a ruby or
sapphire is really corundum (Al2O3) or just
colored glass (SiO2). The spectrum in figure
4 shows the corundum ruby and sapphire, as
well as an emerald with clearly different sig-
nals of the main components Al and Si.
The contents of detectable elements range
from about 10 ppm to a percentage range.
Generally, all elements from Na to U can be
detected but those elements listed in table 1
are typically found in gemstones.
Elements Range of concentration Elements Range of concentration
Mg Matrix [%] Zn 10-3000 ppm
Al Matrix [%] Ga 10-600 ppm
Si Matrix [%] Rb 10-300 ppm
K 10-3000 ppm Sr 10-500 ppm
Ca 10-35000 ppm Zr 10-600 ppm
Sc 10-100 ppm Nb 10-20 ppm
Ti 10-500 ppm Mo 10-100ppm
V 100-2000 ppm Sn 10-200 ppm
Cr 10-18000 ppm Sb 10-100 ppm
Mn 50-13000 ppm Cs 50-2000 ppm
Fe 10-100000 ppm Ta 10-100 ppm
Co 10-100 ppm W 10-50 ppm
Cu 10-15000 ppm Pb 10-2000 ppm
Ni 10-500 ppm Bi 10-10000 ppm
Figure 4: Spectra of ruby, sapphire and emerald
Table 1: Typical elements in gemstones and
their ranges of concentration
Energy [keV]
Impu
lses
[x10
5 ]
5Verification of the Authenticity and Classification of Gemstones Using XRF
The following four figures show correlation
curves of relevant elements. A good correla-
tion between reference concentration and
analytical result can be seen.
Figures 5-8: Correlations of Cr, Fe, Bi and Mn in
emeralds and tourmalines
R² = 0,9927
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6
Anal
ysis
resu
lt [%
]
Reference concentration [%]
Correlation of Cr in Emerald
R² = 0,9982
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
0 0,5 1 1,5 2 2,5 3 3,5 4
Anal
ysis
resu
lt [%
]
Reference concentration [%]
Correlation of Fe in Emerald
R² = 0,9967
0,15
0,2
0,25
0,3
0,35
0,4
0,45
0,5
0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5
Anal
ysis
resu
lt [%
]
Reference concentration [%]
Correlation of Bi in Tourmaline
R² = 0,9903
0
0,2
0,4
0,6
0,8
1
1,2
1,4
0 0,2 0,4 0,6 0,8 1 1,2 1,4
Anal
ysis
resu
lt [%
]
Reference concentration [%]
Correlation of Mn in Tourmaline
6 Verification of the Authenticity and Classification of Gemstones Using XRF
If you want to determine the origin of gem-
stones, you will either require knowledge
about the elemental composition typical for
the region or you will need to build a data-
base.
Figure 9: Elemental distribution of chromium to iron in
emeralds [Source: Guild]
Figure 10: Elemental distribution of iron to chromium in
rubies [Source: Guild]
Figures 9 and 10 show the correlations of
iron to chromium in emeralds and rubies in
different regions of the world. For example,
an emerald from Colombia has a relatively
low iron content compared to emeralds from
Zambia. For rubies, the countries Burma,
Mozambique and Tanzania are shown as
clusters.
7Verification of the Authenticity and Classification of Gemstones Using XRF
PEARLS
What for gemstones is similarly valid for
pearls. Pearls can be cultured, but a saltwater
pearl is much more valuable than a freshwa-
ter pearl due to the effort required to cultivate
it, and it looks very similar.
Determining whether it is a fresh or salt water
pearl can be done with an XRF instrument by
analyzing the manganese content, because
the matrix in both kinds is calcium carbon-
ate (CaCO3) and thus identical. If a pearl is
cultured in fresh water, more manganese is
stored in it than in a saltwater pearl. Fresh
water contains a relatively high amount of
manganese (5 ppb) compared to salt water of
the oceans (0.03 - 2 ppb). This difference can
be detected with ED-XRF as the spectrum in
Figure 11 shows, which is a comparison of
an analysis of a fresh and a salt water pearl.
Figure 11: Comparison on an analysis of a freshwater and a
saltwater pearl
Figure 12: Above: freshwater pearls, below: saltwater pearls
Energy [keV]
Impu
lses
[x10
2 ]
8Verification of the Authenticity and Classification of Gemstones Using XRF
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Summary:
Through the purity and the presence of significant elements, the authenticity and even origin of the gemstones can
be inferred. This helps, for example, in determining the value. Background knowledge about the presence of trace
elements typical for the respective region is required, especially when deciding of the gemstone's origin. In pearls, this
element is essentially manganese, which makes the difference between salt and freshwater pearls. Energy dispersive
X-ray fluorescence analysis (ED-XRF) can be used for all these tasks and can thus allow us to obtain certainty regarding
information on gemstones and pearls and ultimately differentiate natural from synthetic gemstones.