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142 THE JOURNAL OF GEMMOLOGY, 36(2), 2018
FEATURE ARTICLE
What Truly Characterises
a Chameleon Diamond?
An Example of an Atypical
25.85 ct Stone
Emmanuel Fritsch and Aurélien Delaunay
ABSTRACT: We document an exceptionally large, 25.85 ct diamond that shows a slight colour
change but exhibits some atypical properties for chameleon diamonds, including white luminescence
to long- and short-wave UV radiation, as well as a network-like pattern seen in most orientations
with the DiamondView. In considering whether to call this a chameleon diamond, we undertook a
review of available data to compile the properties that are commonly exhibited by these gems. We
found that, in addition to their defining photochromic and thermochromic behaviour, nine charac-
teristics all must be present: long-lasting yellow phosphorescence, a zoned DiamondView growth
pattern showing yellow-green/blue/inert areas, the presence of dominant A aggregates and also some
hydrogen in the infrared spectrum, a continuum of absorption in the visible range related to a very
weak type Ib character, a 480 nm absorption band that is possibly related to trace amounts of oxygen,
a 425 nm absorption band, a weaker absorption band in the red to near-infrared region consistent
with hydrogen-related defects, and traces of nickel detected with photoluminescence spectroscopy.
FEATURE ARTICLE
Figure 1: The 25.85 ct chameleon diamond studied by the authors is shown with its slightly greyish green ‘stable’ colour
at room temperature (left) and its greyish yellow ‘heated’ colour (right). Photos by A. Delaunay.
The Journal of Gemmology, 36(2), 2018, pp. 142–151, http://dx.doi.org/10.15506/JoG.2018.36.2.142
© 2018 The Gemmological Association of Great Britain
THE JOURNAL OF GEMMOLOGY, 36(2), 2018 143
CHAMELEON DIAMONDS
Chameleon diamonds change colour when
heated or left in the dark. Such gems are thus
thermochromic and photochromic, respec-
tively. Obviously, the term chameleon refers
to the reptile known to change colour according to its
environment. The ‘heated’ colour of such diamonds
is at best an attractive orangey yellow, while the other
(‘stable’) colour is much less saturated and darker,
often tending toward greyish green. This unusual colour
behaviour, combined with the attraction for coloured
diamonds in general, makes chameleon diamonds
much-valued collectibles, and gemmological laborato-
ries occasionally issue reports on them. Here, we detail
an unusual example, which poses the questions: What is
the definition of a chameleon diamond? What properties
must a diamond have to be properly named chameleon?
In addition to their collectable and gemmological
aspects, chameleon diamonds are of interest to science
in general. The so-called ‘X-chrome’ materials (X- being
thermo, photo, tribo, electro or some other prefix related
to an excitation that results in a change in colour) have
a great interest in physics (e.g. for optical storage; Irie,
2000). Hence chemists have produced a number of
X-chromes, and physicists attempt to understand the
physical phenomena behind the colour change. From
this perspective, chameleon diamonds represent a super-
lative as they combine two X-chrome behaviours with
a super-material, diamond, also often used by theore-
ticians because of its simple chemistry and structure.
Thus, several studies have attempted to understand
the electronic structure and atomic defects responsible
for this colour behaviour, so far unsuccessfully. If one
could duplicate such behaviour in diamond, this would
be an outstanding X-chrome material as diamond is
already an exceptional substance—even without the
chameleon effect. This would be a good example of
what we call mineralo-mimetism: Nature is capable
of producing minerals with interesting properties that
humans try to duplicate for their own benefit. (This
notion parallels bio-mimetism, but outside the biological
world.) Amusingly, while humans do not entirely under-
stand the mechanism involved in chameleon diamond,
the colour behaviour of the eponymous reptile has only
recently been elucidated (Teyssier et al., 2015).
The French Gemmological Laboratory (Laboratoire
Français de Gemmologie, LFG) recently documented a
25.85 ct chameleon diamond submitted for a grading
report (Figures 1 and 2). This is the largest such diamond
seen at LFG, and it was analysed in detail. Its properties
are somewhat surprising, such that it was not readily
recognised as a chameleon when submitted. Its remark-
able phosphorescence is what prompted the laboratory
to test its change of colour, which was present but subtle.
Many chameleon diamonds have been documented
in the gemmological literature. The largest one is the
31.31 ct oval-cut Chopard diamond acquired by the Swiss
jeweller in 2007 (Fritsch et al., 2004; Chopard, 2014).
Until now, the second largest was a 22.28 ct heart-shaped
stone named ‘the 22 ct Green Chameleon Diamond’
(Moses, 1992; Fritsch et al., 1995). The stone documented
in the present article represents a new second-largest
documented chameleon diamond, which further explains
our interest in scrutinising its characteristics.
METHODS
The diamond was graded using standard procedures,
and its colouration was documented in a Macbeth Judge
II viewing booth using a D65-like illuminator. Lumines-
cence was observed with a Vilber Lourmat VL-6 UV lamp
equipped with 6 W tubes (long-wave 365 nm and short-
wave 254 nm) in a standard cabinet, with the diamond
placed about 7 cm from the lamp. Luminescence images
excited with ultra-short-wave UV radiation (~220–230
nm) were acquired with a DiamondView instrument. A
Fourier-transform infrared (FTIR) absorption spectrum
was obtained with a Bruker Tensor 27 infrared spectrom-
eter with 2 cm–1 resolution, accumulating 2,000 scans to
improve the signal-to-noise ratio. An ultraviolet-visible
(UV-Vis) absorption spectrum was collected with a
Figure 2: Viewed here table-down, the 25.85 ct chameleon diamond displays a progressive change in colour when heated from
ambient conditions (left) to 160°C (right). Photos by A. Delaunay.
144 THE JOURNAL OF GEMMOLOGY, 36(2), 2018
FEATURE ARTICLE
Jasco V-670 spectrophotometer, with a sampling interval
and spectral bandwidth of 1 nm. Photoluminescence
(PL) spectra were obtained at liquid-nitrogen tempera-
ture with a Renishaw inVia Raman spectrometer using
two laser excitations (325 and 514 nm), with a sampling
interval of 0.043 nm in a single scan.
RESULTS
The diamond weighed 25.85 ct and measured approxi-
mately 19.22 × 19.71 × 11.31 mm. Its clarity grade was VS1
due to small feathers on the crown and little chips on the
crown and girdle. The colour changed from Fancy slightly
greyish green to Fancy greyish yellow when heated to
160°C on a chemistry heating plate (again, see Figure 2).
The diamond reverted to its ‘stable’ colour very rapidly
(in less than one minute) when left at room tempera-
ture. In addition to this thermochromic behaviour, the
diamond became yellower after being stored overnight
in a dark safe, demonstrating its photochromism.
Under long-wave UV radiation, the diamond emitted
an intense chalky white luminescence with some vaguely
yellowish and bluish zones (Figure 3). White fluores-
cence is not common in diamond, and often is forgotten
as a possible colour of luminescence. It results from
concurrent blue and yellow emissions which, taken
separately, are the most common colours of fluores-
cence in diamond. When the UV lamp was turned off,
a strong yellow phosphorescence was observed that
decayed slowly (approximately 5 minutes). Weaker white
luminescence and shorter yellow phosphorescence were
also observed with the short-wave UV lamp.
Luminescence images acquired with the DiamondView
revealed the surprising presence of what appeared to be
a dislocation network superimposed on the diamond’s
overall blue emission (Figure 4). The dislocation network
was inert on the surrounding blue matrix, which is exactly
the reverse of what has been observed in many natural
type IIa diamonds. The network appeared somewhat
layered or banded. The pavilion of the stone contained
a distinct zone of yellow-green emission (with inert and
blue areas) along graining in the diamond (Figure 5). This
yellow-green luminescence is somewhat different from the
green emission produced by the H3 centre. Yellow-green/
blue/inert zoned luminescence is characteristic of ‘classic’
chameleon diamonds in the DiamondView.
The FTIR absorption spectrum was typical of type
Ia diamond with high nitrogen content (Figure 6). The
relative proportion of A to B aggregates can be seen by
comparing the bands at 482 cm–1 (A aggregates) and 1010
cm–1 (B aggregates). A comparison of the size of these
two bands indicates that the diamond is type IaA>>B,
like other chameleons described in the past (see, e.g.,
Hainschwang et al., 2005). In addition, some hydrogen-re-
lated absorptions at 3107 and 1405 cm–1 were recorded,
indicating a moderate H content. Hydrogen is always
present in chameleon diamonds, but often at relatively
high concentrations (approaching ‘H-rich’ amounts;
Hainschwang et al., 2005), which is not the case here.
The UV-Vis absorption spectrum consisted of a combi-
nation of several features. An underlying continuum
gradually increased from the red towards the violet
region. On it was superimposed a complex, broad band
located in the red to near-infrared region starting at
~580 nm. This feature is postulated to be related, at
least in part, to the thermochromic behaviour (Fritsch
et al., 1995). Considering its spectral range, as well as its
complex multi-component nature, it is similar to absorp-
tion features related to hydrogen (Fritsch et al., 2007a). In
addition, there was a weak feature at 425 nm and a broad
Figure 3: The 25.85 ct diamond luminesces intense chalky white to long-wave UV radiation (left) and weaker white to short-wave
UV (centre). The unusual yellow phosphorescence (right) is characteristic of chameleon diamonds. Photos by A. Delaunay.
THE JOURNAL OF GEMMOLOGY, 36(2), 2018 145
CHAMELEON DIAMONDS
band at 480 nm (Figure 7; Hainschwang et al., 2005).
The 480 nm band is known to occur in yellow-orange
diamonds and is always present in chameleons, along
with its 425 nm companion (Chabert and Reinitz, 2000).
The 480 nm band is possibly due to traces of oxygen
(Hainschwang et al., 2008). The N3 centre at 415 nm
was also observed, and has been documented previously
in chameleon diamonds (Chabert and Reinitz, 2000).
Photoluminescence spectra obtained with 514 nm
excitation revealed Ni-related emissions at 700.5, 793.6
and 881.3 nm (Figure 8), which have been documented
previously in typical chameleon diamonds (Hainschwang
et al., 2005). In addition, there was a weak peak at 884.7
nm, which might be part of the well-known Ni-related
doublet in the 883 nm region. A broad band at 630 nm
was seen in the 514 nm PL spectrum, as well as other
weaker underlying broad bands. The 630 nm band
was previously documented in type IaA/Ib orange to
Figure 5: A yellow-
green-luminescing zone
with graining is seen
on the pavilion of the
chameleon diamond
with the DiamondView,
together with black
(inert) and blue zones.
Photomicrograph by
A. Delaunay; image
width 9 mm.
Figure 4: In the
DiamondView, the
25.85 ct diamond
shows an atypical
dislocation network.
Photomicrograph by
A. Delaunay; image
width 9 mm.
146 THE JOURNAL OF GEMMOLOGY, 36(2), 2018
FEATURE ARTICLE
orange-yellow diamonds that exhibit the 480 nm absorp-
tion (Hainschwang et al., 2005). Other broad bands were
present as well, roughly at ~700 and 800 nm. No GR1
(741 nm) feature was observed for this stone, whereas
all previous chameleon diamonds studied by the authors
displayed a weak GR1. In the PL spectrum obtained
with 325 nm excitation, the main broad band was at
~560 nm. Emission peaks of the N3 centre at 415 nm
and the H3 centre at 503 nm were otherwise the most
prominent features of this spectrum (Figure 9), and there
also appeared to be an underlying broad, intense feature,
possibly centred in the red region.
DISCUSSION: WHAT IS COMMON
TO ALL CHAMELEON DIAMONDS?
The fact that this diamond, as well as some others before
it, at first eluded proper identification as chameleons
(see, e.g., Fryer, 1981) proves that such diamonds cover
a range of possible properties. This begs the question:
What combination of properties are always present in
chameleon diamonds that might be considered unique
to this variety? We will exclude the ‘reverse chameleon’
variety defined by Hainschwang et al. (2005), which
has not been documented again since. The following
Wavelength (nm)
Absorbance
Figure 7: The UV-Vis spectrum
of the chameleon diamond taken
at room temperature displays the
N3 centre at 415 nm, a weak band
at 425 nm, a broad band at 480
nm and another broad band in the
red to near-infrared region. These
features are all superimposed on a
slight absorption continuum that
rises towards the ultraviolet. The
spectrum was obtained through the
girdle, for an approximate optical
path length of 19.5 mm.
UV-Vis Spectrum
Wavenumber (cm–1)
Absorbance
Figure 6: The FTIR spectrum of
the chameleon diamond reveals
a high concentration of nitrogen
in A-aggregate form (482 cm–1),
a small amount of B aggregates
(1010 cm–1) and a low content of
hydrogen (3107 and 1405 cm–1).
FTIR Spectrum
THE JOURNAL OF GEMMOLOGY, 36(2), 2018 147
CHAMELEON DIAMONDS
Wavelength (nm)
Intensity
Figure 9: Distinct emissions from
N3 and H3 centres are seen in the
PL spectrum of the chameleon
diamond obtained with 325 nm
laser excitation at liquid-nitrogen
temperature.
PL Spectrum
Wavelength (nm)
Intensity
Figure 8: Photoluminescence
spectroscopy of the chameleon
diamond with 514 nm laser
excitation at liquid-nitrogen
temperature reveals numerous
emissions related to Ni.
PL Spectrum
514 nm Excitation
325 nm Excitation
discussion is based on the authors’ documentation of
approximately 100 chameleon diamonds, often during
short loans. Thus, not all properties could be documented
for all of them, depending on the time and instrumen-
tation available. However, a consistent pattern appears
from this intermittent study over almost 30 years.
Again, the defining property of classic chameleon
diamonds is their combined thermochromic and photo-
chromic behaviour. They must change colour when left
in the dark for several hours or after being moderately
heated (160–200°C). The return to the original colour
is generally within a minute after being brought from
darkness to normal lighting conditions, or after being
left at room temperature in normal lighting after heating.
There is a range of possibilities for the two hues observed.
Overall, the diamond changes from a tint containing some
green to another containing yellow (see, e.g., Koivula and
Kammerling, 1991; Fritsch; 1998; Hofer, 1998). Figure 10
illustrates a diamond showing a distinct colour change, in
contrast to the subtle change in the ~25 ct gem reported
here. Brown and grey also are common colour compo-
nents of chameleon diamonds (Breeding et al., 2018).
Hofer (1998) described some chameleon colours as ‘olive’,
an imprecise term, which consists of a mixture of colours
148 THE JOURNAL OF GEMMOLOGY, 36(2), 2018
FEATURE ARTICLE
including green, with the addition of brown, yellow and
grey (Hofer, 1998; Gaillou and Rossman, 2017).
Thermochromic behaviour has been associated with
‘chameleon’ diamonds since at least 1867 (Figuier,
1867). At that time, however, it was described as a
reversible change from near-colourless to pink. This
curious property was subsequently mentioned in one
of the least-known books by Jules Verne, The Star of the
South (L’Etoile du Sud), published in 1884 (Fritsch, 2016).
In the book, the ‘chameleon’ diamond changed from
black to pink. The first modern definition of a chameleon
diamond that we could find is in the first edition of The
Diamond Dictionary (Copeland et al., 1960).
The characteristic that often alerts a gemmologist that a
diamond might be a chameleon is yellow phosphorescence
excited by a typical short-wave UV lamp. The phosphores-
cence may be of varying intensity, but it is characteristically
quite prolonged (i.e. over several minutes if observed in
total darkness). However, this property alone does not
correlate to the chameleon character (Moe et al., 2017).
It is just a convenient method to highlight diamonds that
might be chameleons, as this yellow phosphorescence is
otherwise rare in diamond. It is generally accompanied by
a strong yellow emission (which was present in the 25.85 ct
stone together with blue N3 emission) when excited
with long-wave UV radiation (Breeding et al., 2018).
This yellow emission, as mentioned above, is indirectly
related to the 480 nm band in chameleon diamonds
(explained below). It has been described as a broad band
of ‘moderate’ intensity centred at ~560 nm (i.e. using
fluorescence spectroscopy; Eaton-Magaña et al., 2007).
Sometimes a sharper feature at about 507 nm accom-
panies this broad emission band (Byrne et al., 2015).
In the small number of cases (about a dozen) when
chameleon diamonds were observed by the authors in
the DiamondView, they all showed a patchy yellow-green
and blue emission with some inert areas (Figure 11).
Characteristics of the colour distribution are consistent
with the blue zones resulting from octahedral growth,
whereas the yellow-green zones (often with curved
boundaries) correspond to cuboid growth. Although
these two growth habits are often associated in diamond,
this specific pattern seems unique to chameleons. In
addition, we have never seen such an appearance in
the DiamondView for non-chameleon diamonds, for
which we have carefully documented hundreds, and
possibly thousands, of patterns. Furthermore, we were
able to identify two chameleon diamonds through their
DiamondView pattern that had not been recognised
previously as such.
The zoning described above is a curious aspect of
chameleon diamonds. Despite this rather obvious and
distinct growth zoning, both the body colour distribu-
tion and the colour change appear homogeneous when
observed with the unaided eye.
Regarding their infrared spectra, chameleon diamonds
are consistently type Ia, with low-to-moderate concen-
trations of nitrogen. In all cases, A aggregates largely
dominate, even in some rare diamonds that also contain B
aggregates; absorption from the latter is never as strong as
that of the A aggregates (e.g. Panjikar and Panjikar, 2015).
In the infrared region, they always show low-to-moderate
absorption of the 3107 cm–1 system (Fritsch et al., 2007a),
some even reaching the ‘H-rich’ level (Fritsch, 1998).
In the UV-Vis region, chameleon diamonds all show
a combination of: (1) a continuum of absorption rising
Figure 10: This 4.85 ct chameleon diamond shows a pronounced colour change (here, with heating) from greyish green to
orangey yellow. Photo courtesy of SanaDiam, Antwerp, Belgium.
THE JOURNAL OF GEMMOLOGY, 36(2), 2018 149
CHAMELEON DIAMONDS
towards the UV, starting at ~550 nm; (2) an often weak
but distinct 480 nm broad band and its companion band
at ~425 nm; and (3) a broad band in the red to near-in-
frared region. Of course, the individual components are
easier to identify if the spectrum is obtained at liquid-ni-
trogen temperature. The N3 absorption is common
but sometimes absent, so its presence does not seem
mandatory for chameleon diamonds. The change of colour
with heat corresponds in our experience to a slight shift
of the continuum towards higher wavelength, combined
with a decrease of the red to near-infrared feature. This
makes the green transmission window disappear, together
with the green colour component. The resulting spectrum
is typical of some orangey yellow diamonds, with both the
shifted continuum and the 480 nm feature contributing
to the yellow component. In our view, the slight decrease
of the 425 and 480 nm bands plays almost no role in the
change of colour. By contrast, Khan et al. (2010) as well
as Gaillou and Rossman (2017) attributed the change in
colour to variations in 480 and 800 nm absorptions, not
taking into account the underlying continuum.
The absorption continuum, starting at ~560 nm and
increasing towards the ultraviolet, might be attributed to
a very weak type Ib component, which can be resolved
in the infrared spectrum of many chameleon diamonds
(Hainschwang et al., 2005), but not all. A type Ib visible-
range absorption continuum can be found even in the
absence of the 1344 cm–1 infrared absorption typical of
type Ib diamond (Hainschwang, 2014). As those infrared
features are slight, they might easily be missed or not
looked for in some stones. The temperature behaviour
of the continuum, which is slightly thermochromic, is
consistent with that of type Ib diamonds. A band at
425 nm is found in type Ia diamonds known to contain
hydrogen (Fritsch et al., 2007b), but this band in
chameleon diamonds might be due to a different defect,
given that it appears to be a ‘companion’ of the 480
nm band (Collins, 1982). The 480 nm band typically
found in all chameleons (Chan et al., 2015) is believed
to be related to the presence of oxygen (Hainschwang
et al., 2008, and references therein). In addition, three
non-chameleon yellow diamonds displaying the 480 nm
absorption contained ESR-active defects linked to oxygen
(unpublished data of Solveig Felton at the University
of Warwick in 2009). The red to near-infrared feature
seems to vary in shape from chameleon to chameleon,
probably because it is a combination of several features,
possibly involving other broad bands in the near-infrared
(Fritsch et al., 2007a). This pattern might be related to
the presence of hydrogen-related defects, possibly linked
to A aggregates (Goss et al., 2011).
In laser-excited photoluminescence spectra, no single
peak seems absolutely characteristic, but the ~701 nm
system appears very common. As with some other
emissions documented in chameleon diamonds, this
one is related to traces of nickel. Thus Ni traces might
be necessary to obtain the chameleon effect. Nickel
was detected by energy-dispersive X-ray fluorescence
spectroscopy in some chameleon diamonds (as well as
‘canary’ diamonds) exhibiting the 480 nm absorption
(De Weerdt and Van Royen, 2001). In essence, when
Ni is looked for in a chameleon diamond, it is found.
Nickel is actually not so rare an impurity in natural
diamond, particularly in natural type Ib diamonds
Figure 11: This 0.27 ct chameleon
diamond shows a DiamondView
pattern typical for chameleons, with
a combination of yellow-green, blue
and inert zones. The straight blue-
luminescing growth zones represent
octahedral growth. The curved
boundary between the inert and
yellow-green zones is reminiscent of a
cuboid sector. Photomicrograph by
A. Delaunay; image width 6.4 mm.
150 THE JOURNAL OF GEMMOLOGY, 36(2), 2018
FEATURE ARTICLE
(Hainschwang et al., 2013), even outside the chameleon
variety. However, it was not always detected in the past
in chameleon diamonds (Fritsch et al., 2007b), simply
because the necessary instrumentation was not available
or, alternatively, it was not looked for.
The list below summarises the features documented
in all the chameleon diamonds that we have examined,
in addition to their defining photochromic and thermo-
chromic behaviour: They change from a greener to a
yellower colour when left in the dark for several hours,
or after being moderately heated (160–200°C) for about
a minute.
• Long-lasting (several minutes) yellow phosphores-
cence to short-wave UV radiation
• Zoned growth pattern, with a combination of blue
octahedral and yellow-green cuboid growth, seen in
the DiamondView
• Type IaA dominant, plus a type Ib component
• Some hydrogen, as indicated by the 3107 cm–1 system
• Absorption continuum in the UV-Vis range, related to
the type Ib character
• 480 nm band, possibly related to an oxygen impurity
• 425 nm band, possibly related to hydrogen
• Broad feature in red to near-infrared region, likely
H-related
• Traces of Ni
We thus believe that all of these features must be taken
into account to explain the thermochromism and photo-
chromism shown by chameleon diamonds. This list,
which is quite long, is difficult to reconcile with a simple
electronic structure relating to the defects responsible for
chameleon behaviour. Various authors have suggested
different explanations (Massi et al., 2006; Fritsch et al.,
2007b; Butler et al., 2017; Byrne et al., 2018), but the
complex nature of chameleon diamonds, combined
with the nine characteristics listed above, makes their
optical behaviour difficult to model. Numerous defects
are present, associated with several impurities (nitrogen,
in particular N-N or A aggregates, as well as hydrogen,
nickel and oxygen), and it is difficult to assess if all
necessarily play a role. However, the consistency of our
observations on chameleon diamonds for almost 30
years leads us to believe that all these factors are signif-
icant. The list of nine characteristics should thus be
taken into account in any forthcoming model for the
chameleon-inducing electronic structure in diamond.
CONCLUSION
The study of an exceptionally large, 25.85 ct diamond
with some unusual properties for chameleon diamonds
prompted the question of what actually defines this
unusual variety. By compiling the data provided by the
available literature and combining it with the authors’
own experience, it appears that the specifications are
more numerous than expected. Indeed, the nine charac-
teristics identified in this article appear to be necessary
in combination. This likely explains why the origin of
the chameleon behaviour in diamond remains a mystery.
Even though the 25.85 ct stone documented in this
article had some unusual properties, it did have all of
the characteristics required for a chameleon diamond.
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The Authors
Dr Emmanuel Fritsch FGA
Institut des Matériaux Jean Rouxel, University of
Nantes, CNRS, 2 rue de la Houssinière, BP 32229,
44322 Nantes Cedex 3, France.
Email: emmanuel.fritsch@cnrs-imn.fr
Aurélien Delaunay
Laboratoire Français de Gemmologie, 30 rue Notre
Dame des Victoires, 75002 Paris, France
