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2 BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015
Blue spinels are mined in Sri Lanka, Tanzania,
Myanmar, Pakistan, and Vietnam (Shigley and
Stockton, 1984; Delaunay, 2008; Pardieu and
Hughes, 2008). Vietnam’s two major spinel deposits,
Luc Yen and Quy Chau, were discovered at the end
of the 1980s. The Luc Yen deposits have mostly been
mined since the 1990s (Pardieu and Hughes, 2008;
Senoble, 2010). This area, known for its gem-quality
ruby, red spinel, and sapphire (Webster, 1994;
Hauzenberger et al., 2003; Long et al., 2004; Senoble,
2010; Huong et al., 2012), has also been a notable pro-
ducer of vivid blue spinels since the 2000s (figure 1).
Bright, saturated blue gems are very popular, as evi-
denced by the classic appeal of sapphire and the more
recent trend of Paraíba tourmaline and bright blue
apatite in the same color range. Therefore, the bright
blue color of some spinels has increased the popular-
ity of this gem overall (Delaunay, 2008; Senoble,
2010). In this article, we investigate the gemological
characteristics of Vietnamese blue spinels and the ge-
ology of the deposits to gain a better understanding
of this gem source.
LOCATION AND ACCESS
The Luc Yen district is located in the Yen Bai
province, in the north of Vietnam. Luc Yen’s capital
of Yen The (22°6′38.84″N, 104°45′57.80″E) is a five-
or six-hour drive from Hanoi on a 160 km expanse of
good road. All of the district’s blue spinel mining
sites lie within 20 km of Yen The. Several hours of
walking or biking are needed to access these mines.
The blue spinel deposits are Bai Gou, May Trung, Bai
Son, Bah Linh Mot, Khe Khi, Kuoi Ngan, Khao Ka,
Lung Thin, Lung Day, Khin Khang, and Chuong Tran
(figure 2). Of these, only May Trung, Bai Son, and Bah
Linh Mot are primary deposits; the others are second-
ary placer deposits. Bai Son, reported by Senoble
BLUE SPINEL FROM THE
LUC YEN DISTRICT OF VIETNAM
Boris Chauviré, Benjamin Rondeau, Emmanuel Fritsch, Phillipe Ressigeac, and Jean-Luc Devidal
FEATURE ARTICLES
The Luc Yen district of northern Vietnam is a very productive gem province and the leading source of
vivid blue spinel. This study characterizes the origin and gemological properties of these spinels, espe-
cially the cause of their unusually bright color, which is directly related to their value. Chemical and
spectroscopic analyses indicated that the blue color is due to cobalt (Co2+), with some iron contribution.
Petrographic examination identified the context of the gem’s formation, which appears to be linked to
intense metamorphism during successive orogenies. The carbonate platforms in the ancient Paleo-Tethys
Ocean were sandwiched and highly deformed during this orogeny, leading to marble and spinel for-
mation. The authors propose that the cobalt (and to a lesser extent the iron) necessary for the blue color
were transported by fluids during metamorphism of the sedimentary sequence.
See end of article for About the Authors and Acknowledgments.
GEMS & GEMOLOGY, Vol. 51, No. 1, pp. 2–17,
http://dx.doi.org/10.5741/GEMS.51.1.2.
© 2015 Gemological Institute of America
In Brief
• Over the past two decades, Luc Yen, Vietnam has be-
come a notable source for blue spinel.
• Vietnamese blue spinel may have resulted from the
involvement of evaporitic rocks during post-collision
metamorphism.
• Cobalt (Co2+) is the main chromophoric element in
blue spinel, though iron (Fe2+) is also a factor.
(2010), was no longer being mined as a primary de-
posit during our visit in February and March 2012.
May Trung is divided into two sites located about
150 meters from each other: a marble cliff that is
mined for red and lavender spinels, and a second site
that is mined for blue spinel from a vein in marble.
GEOLOGY
The rich tectonic history of Southeast Asia is inher-
ited from several deformation episodes related to the
closure of the Paleo-Tethys Ocean and, later, to the
Himalayan orogeny. The geology of northern Viet-
nam is dominated by metamorphic rocks inherited
from these two major orogenic events. The first one,
the Indosinian orogeny, led to the collision of the
main shields (Yangtze and Indochina) during the
Permo-Triassic at about 240–245 Ma (Kušnír, 2000;
Lepvrier et al., 2008; Huong et al., 2012). In the later
orogeny, the Himalayan collision during the Tertiary
period, the terrains were strongly reworked. These
terrains are primarily composed of metamorphic
rocks, mainly medium-grade mica schist, marble,
and granulitic gneisses (Kušnír, 2000; Leloup et al.,
2001; Hauzenberger et al., 2003).
Northern Vietnam has been studied extensively
to understand how a continental collision (in this
case, between India and Eurasia) induced crustal
wedges to extrude laterally into the surrounding
BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015 3
Figure 1. The Luc Yen
district of Vietnam has
become a major source
of top-quality blue
spinel, including these
two rough crystals (45
and 70 ct) and the 5 ct
faceted stone. Photo by
J.B Senoble; © Senoble
& Bryl.
plates. (Tapponnier et al., 1982, 1990; Leloup et al.,
1995, 2001; Jolivet et al., 2001; Anckiewicz et al.,
2007). During the Oligo-Miocene (from 35 to 17 Ma),
the Indo-Eurasian collision induced strong rock de-
formation over all of Southeast Asia. The Indochi-
nese block was extruded toward the southeast, and
this induced the Red River Shear Zone, extending
from the Tibetan plateaus to the China Sea for more
than 1,000 km (Jolivet et al., 2001; Leloup et al., 2001;
Hauzenberger et al., 2003; Anckiewicz et al., 2007).
The Yen Bai province is formed by two different ge-
ological units separated by a fault that is part of the
Red River shear zone. To the northeast lies the Lo
Gam zone, and to the southwest the Day Nui Con
Voi range (figure 3).
All of Luc Yen’s gem deposits are located in the
Lo Gam zone (again, see figure 3). The structure of
this unit results from the deformation of the Hi-
malayan orogenesis superimposed on the preexisting
Indosinian structure (Garnier et al., 2002, 2005). The
Lo Gam formation consists of a sedimentary series
metamorphosed into marble, gneiss, calc-silicates,
micaschist, and amphibolite. These metamorphic
rocks are sometimes intruded by granitic and peg-
matitic dykes (Leloup et al., 2001; Garnier et al.,
2005, 2008). The marbles are mainly calcitic and in-
terlayered with Al-, V-, and Cr-rich amphibolites.
Blue spinel is found in a layer of marble more than
500 meters thick. It occurs in discontinuous series of
lenses, tens of millimeters thick and meter-sized in
length, roughly following the regional foliation. These
marble lenses are remarkably rich in forsterite (mag-
nesian olivine). The gem is often associated with cal-
cite, forsterite, pargasite (sodi-calcic amphibole),
sulfides, and chlorites (magnesian chlorite and
clinochlore). Remarkably, blue spinel in these primary
deposits is not associated with ruby or red spinel.
MINING
The three primary deposits at May Trung (22°1′48.9″
N, 104°48′42.7″E), Bai Son (21°59′47.3″N, 104°40′9.9″
E), and Bah Linh Mot (22°1′23.7″N, 104°48′42.8″E)
are located on a mountain range composed of marble,
standing about 600 meters high. Each site is mined by
4 BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015
Luc Yen District
VIETNAM
Bangkok
Hanoi
LAOS
THAILAND
CAMBODIA
Bai Gou/Chuong Tran
May Trung
Khin Khang
Bah Linh Mot
Anh Pha
Lung Thin
Bai Son
Lung Day
Kuoi Ngan
Yen The
Khao Ka
Thac Ba Lake
Primary Secondary
Blue Spinel Deposit
Road
Major cities/towns
Khe Khi
2 km
N
Figure 2. Most Viet-
namese blue spinel
deposits are confined to
a 30 km² area in the Luc
Yen district of northern
Vietnam. The circles
represent placer deposits,
while diamonds indi-
cate primary deposits in
marble.
a handful of locals, mainly farmers trying to earn extra
income. The blue spinel is extracted from the marble
using hand tools (figure 4, top) and a jackhammer.
Secondary deposits (figure 4, bottom left and bot-
tom right) yield most of the blue spinel production.
Some lie in the valley to the east of the spinel-rich
mountain range. These include Kuoi Ngan (22°0′7.8″
N, 104°50′41.1″E); Khao Ka (21°59′6.5″N, 104°50′52.5″
E); Lung Thin (22°0′12.8″N, 104°49′31.5″E); Lung Day
(21°59′51.3″N, 104°49′23.3″E); and Khin Khang
(22°1′46.7″N, 104°50′9.3″E). Khe Khi (22°1′36.8″N,
104°48′41.2″ E) and Bai Gou (22°4′43.2″N, 104°47′5.5″
E) are located in the mountain in a small secondary
basin. Miners use a water hose and a sluice to sort the
gem-bearing gravels (figure 4, bottom left). Some sec-
ondary deposits are localized in karst caves inside mar-
ble (figure 4, bottom right). Heavy gravels are washed
and sorted inside the cave and brought up to the sur-
face, where they are sorted under daylight. In second-
ary deposits, blue spinel is found together with ruby,
red spinel, sapphire, tourmaline, and occasionally gold.
PRODUCTION AND DISTRIBUTION
In secondary deposits, blue spinel is a by-product of
ruby and red spinel mining. Even so, some large
parcels contain more than a thousand carats of mil-
limeter-sized, very saturated blue spinel (see Pardieu,
2012). Some dark grayish blue stones weighing ap-
proximately 5 ct have been faceted, but far fewer
than ruby and red spinel.
Blue spinels from primary deposits are found in
two different forms. Usually miners encounter them
in “pockets” as centimeter-sized crystals, occasion-
ally with a pleasing, well-defined octahedral shape
BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015 5
Figure 3. This geological map of Luc Yen shows two different geological formations: the Day Nui Con Voi Range in
the southwest and the Lo Gam zone in the northeast. The blue spinel deposits are located in the Lo Gam zone.
Adapted from Garnier (2003) and Long et al. (2004).
An Lac
Cong Quan
Minh Xuan
Tan Lap
Phuc Loi
Quang Minh
Trung Tam
Tan Nguyen
Khe Nhan
Ngoi A
Tan Linh
to Lao Cai
Luc Yen
Khoan Thong
Minh Tien
An Phu
Khe Nhan
Yen Thai
Bai Da Lan
Mong Son
Truc Lau May Trung
Khe Khi
Bah Linh Mot
Lung Thin
Bai Son
Lung Day
Khao Ka
Kuoi Ngan
Khin Khang
Bai Gou / Chuong Tran
8 km
104º4022
10
104º50
22
00
Cenozoic sediment
Ngoi Chi & Nui Voi Formation : gneiss, schist
with marble and amphilobite lense.
Tan Huong granite complex
Can an complex : gabbo, diorite
Bao Ai complex : pyroxenite, homblendite
Quaternary sediments
Dai Thi Formation : quartz-mica-feldspath scist,
quartz-biotitsericite schist, quartite
An Phu Formation :
Upper Proterozoic - Lower Cambrian : calcitic
marble. dolomitic marble with phlogopite-
graphite-margarite
Thac Ba Formation :
Upper Proterozoic - Lower Cambrian : micashist,
quartz-biotite or muscovite schist, gneiss,
migmatite, marble, quartzite
Nui Chua complex : olivine or pryoxene or
amphibole gabbro
Phia Bioc complex : biotite granite, pegmatite
and aplite
Phia Ma complex : hornblende-garnet to
pryoxene granosyenite
Ruby depositPrimary Secondary
Lo Gam zone
Day Nui Con Voi Range
fault
river
road
Blue Spinel Deposit
with some portion containing gem material, or as ag-
gregates of small octahedral crystals of varying qual-
ity. These are broken down or cobbed by miners to
extract a small amount of gem spinel. These gem
blue spinel can reach 5 ct, but they are often frac-
tured. Spinels are also found as millimeter-sized oc-
tahedra or twins in the marble.
The blue spinel’s hue, tone, and saturation vary
from one deposit to the next. In the secondary Bai Gou
deposit, the crystals have a very dark blue color and
often reach 10 ct. In Chuong Tran and Bai Son, the
spinel has a bright blue color (Senoble, 2010; Overton
and Shen, 2011) and can reach 5 ct. Crystals from May
Trung and Khe Khi have a very saturated cobalt blue
color but are quite small (rarely larger than 1 ct).
Millimeter-sized blue spinels from the primary
6 BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015
Figure 4. Gem-quality
blue spinels are ex-
tracted from primary
deposits by locals using
hand tools, as shown in
Bai Nua Doi (top). Sec-
ondary deposits are ex-
ploited in the valley
with a sluice box to sort
the minerals according
to their density (bottom
left). In the karstic envi-
ronment, caves trap the
gem-rich gravel (bottom
right). These special
secondary deposits are
also processed with
sluice boxes. Photos by
Boris Chauviré.
deposits are often kept in marble so that the whole
piece can be carved. This is done directly at the min-
ing site, and the carvings are taken down to the val-
ley to be sold as decorative pieces.
MATERIALS AND METHODS
Sample Collection. In early 2012, two field trips were
organized to collect samples and map the main blue
spinel deposits within the Luc Yen district. The first
expedition enabled us to visit most of the corundum
and spinel deposits and to understand the geology of
this area. We also made contact with local merchants
and miners for the second expedition just one month
later. On the second field trip, we visited only blue
spinel deposits and collected whole rock and gem-
bearing samples. Most of the rock samples were col-
lected at the mining site. Unfortunately, we did not
observe gem samples at the mining sites. All gem
blue spinels for spectroscopic and gemological meas-
urements were procured from several local mer-
chants in Yen The.
Materials. From the 55 carats of blue spinel we col-
lected from local merchants, six representative rough
crystals were selected and prepared in parallel-win-
dow plates for gemological and spectroscopic inves-
tigation. The least included and fractured samples
were chosen for spectroscopic analysis. A polished
window was prepared on each one to facilitate gemo-
logical and microscopic examination. One additional
sample similar to SATBLU1 in color and saturation
(labeled SATBLUchem) was prepared as a polished
section for laser ablation–inductively coupled
plasma–mass spectrometry (LA-ICP-MS) chemical
analysis. Four additional samples from a later field
trip by author EF were added to complete this study.
These were also purchased from local merchants.
The spinels were divided into three different parcels
according to their color category (detailed in “Gemo-
logical Characteristics” below). The sample names
consisted of the color category (GREBLU, SKYBLU
and SATBLU) followed by a number; see table 1.
Moreover, 73 rock samples were collected in the field
from 11 different mining sites. From these, 19 thin
sections were prepared for petrographic examination.
Methods. Gemological Properties. Specific gravity
was measured hydrostatically with a Mettler Toledo
JB703-c/FACT (with a precision of 0.001 ct). Internal
features were observed with a standard gemological
microscope. Refractive index was measured with a
PФ-Іrefractometer with Rayner SVLS orange light.
All spinel samples were observed under a 6 W A-
Krüss Optronic 240 UV light, and we also tested their
Chelsea filter reaction. Color was documented under
normalized daylight (D65) and a normalized incan-
descent light (A).
Spectroscopic Measurements. UV-Vis-NIR absorp-
tion spectra of each sample (window plates) were
taken with a Cary 5G Varian spectrophotometer in
the 200–1500 nm range with a sampling interval of
1 nm and a spectral bandwidth of 1 nm maximum
(sampling and spectral bandwidth were sometimes
reduced to 0.25 nm to obtain better resolution).
Raman spectra were collected on gem samples using
both a Jobin-Yvon Labram with a 514 nm, 50 mW
laser excitation, and a Jobin-Yvon Spex Horiba
T64000 with a 647 nm, 50 mW laser excitation. The
spectral range extended from 40 to 1500 cm–1 with a
two-second exposure.
Chemical Composition. LA-ICP-MS chemical analy-
sis was conducted at Blaise Pascal University (Cler-
mont-Ferrand, France) using an Agilent 7500
spectrometer with a Resonetics M-50E laser (193 nm
ablation wavelength, 5 Hz frequency with an energy
between 10 and 12 J/cm2). For these analyses, four in-
dentations (about 73 µm in diameter) were ablated on
each sample, and 27Al was used as the internal stan-
dard. Data was processed with the GLITTER 4.4.2
software. To complement these analyses, we used a
RIGAKU NEX CG energy-dispersive X-ray fluores-
cence (EDXRF) spectrometer operating at 25 kV and
0.10 mA. The detection limit for the major elements
(Al, Mg) is about 0.1 wt.%, and below 0.01 wt.% for
the minor elements. Each sample was measured for
90 seconds.
Petrographic Examination. Thin sections of rocks
were observed with a standard Wild Makroscope
M420 petrographic microscope, and a JEOL JSL-5800
LV scanning electron microscope (SEM) operating at
20 kV and 0.3 nA electron beam, with a 37° take-off
angle of the detector. Mineral compositions of the
samples and their inclusions were first determined
by energy-dispersive spectroscopy (EDS) using an
IMIX-PTS detector. This detector uses a high-resolu-
tion (115 eV) Ge crystal and an ultrathin polymer
window, detecting elements ideally down to boron,
if it is a major component of the material. The cali-
bration standards used were either pure elements or
simple compounds. The PGT software applies phi-
rho-z data correction for the effect of X-ray absorp-
BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015 7
tion in the analyzed material, taking into account all
the matrix effects. Oxygen was calculated from the
spectrum, not based on stoichiometry.
GEMOLOGICAL CHARACTERISTICS
Visual Appearance. We separated the spinel samples
into three categories according to their color descrip-
tions:
• SATBLU samples: medium to medium dark
tone, strong to vivid saturation, and blue to vi-
oletish blue hue
• SKYBLU samples: medium light to very light
tone, strong to very vivid saturation, and blue
hue
• GREBLU samples: medium light to light tone,
grayish to slightly grayish saturation, and blue
to bluish violet hue
All of the rough samples were slightly fractured
and contained very few inclusions. Color was homo-
geneous in each stone, and most showed a subtle
color change from blue under daylight-equivalent
normalized light (D65) to violetish blue under incan-
descent light (see table 1). The authors avoid the
commonly used term “color shift” (Senoble, 2010),
which Manson and Stockton (1984) defined in gar-
nets while observing the combination of two color
phenomena, nowadays identified separately: classical
color change with lighting, and Usambara effect
(change of color with thickness). We observed that
the color change is more pronounced in stones with
a more saturated color. While examining numerous
parcels in Yen The, we observed that most of the
grayish blue spinel—and some of the very saturated
blue material, contrary to our other observations—
did not show any color change.
Blue spinel from secondary deposits (except Khe
Khi) is rounded and can reach several tens of carats.
In Khe Khi, blue spinels are found as millimeter-
sized octahedra.
Optical and Physical Properties. The samples’ refrac-
tive index ranged from 1.711 to 1.718, and their spe-
cific gravity was from 3.578 to 3.673. They were
isotropic, with no anomalous double refringence, and
inert under both short- and long-wave UV light. Under
the Chelsea filter, all the samples appeared pink to red
(see table 1 for details). We observed that the darker
the spinel, the redder the Chelsea filter reaction.
Microscopic Characteristics. Conchoidal fractures and
“fingerprint” healed fractures were often present in
our samples (figure 5, left). Some showed elongated
tubes, while others contained groups of parallel tubes.
We observed birefringence in some of these tubes,
8 BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015
TABLE 1. Characteristics of gem blue spinels from Luc Yen, Vietnam.
SKYBLU1
2.076
10.8 x 5.2
x 4.3
Khao Ka
1.712
3.583
Pink-
orange
SKYBLU2
0.91
7.6 x 4.6
x 2.7
Khao Ka
1.712
3.584
Pink-
orange
SKYBLU3
0.24
3.8 x 2.7
x 1.5
Unknown
1.714
3.594
Red
SKYBLU4
0.18
2.9 x 2.4
x 0.5
Unknown
1.710
3.596
Red
GREBLU1
4.076
14.9 x 7.1
x 4.1
Bai Son
1.718
3.578
Pink-
orange
GREBLU2
2.538
9.8 x 5.4
x 2.8
Bai Son
1.713
3.598
Pink
GREBLU3
2.863
9.7 x 5.5
x 3.1
Bai Son
1.711
3.583
Pink
SATBLU1
0.28
2.8 x 1.1
x 2.1
Khe Khi
1.712
3.410
Red
SATBLU2
0.23
3.9 x 2.7
x 1.4
Unknown
1.716
3.673
Red
SATBLU3
0.09
3.1 x 1.9
x 1.4
Unknown
1.714
3.645
Red
Samples
Photo
(normalized
daylight)
Photo
(normalized
incandescent
light)
Weight (ct)1
Dimensions
(mm)2
Origin
Refractive index
Specific gravity
Chelsea
reaction
1For SKYBLU4 and SATBLU1, the weight is the sum of the weights of the pieces from the sample.
2For SKYBLU4 and SATBLU1, the dimensions are an average of the measurements of each piece from the sample.
which suggested that they consisted of an anisotropic
solid phase (figure 5, middle and right). Black, opaque,
irregular to hexagonal crystal inclusions less than 1
mm, reminiscent of graphite, were also found in some
samples (figure 5, middle and right). GREBLU1 was
the only sample that had yellowish fractures covered
by red crystals (probably ferric oxide hematite).
PETROGRAPHY AND CHEMISTRY OF HOST
ROCKS
Minerals. The marble that hosts blue spinel is
mainly composed of calcite (sometimes magnesian)
and dolomite. The major additional phases are
olivine and pargasite (figure 6). Several accessory
phases were identified using the petrographic micro-
BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015 9
Figure 5. Blue spinels (here, GREBLU3) often show healed fractures (fingerprints, left), and some samples contain
irregular opaque black crystals associated with elongated tubes (center and right). Photos by Boris Chauviré; field
of view 1 mm (under daylight equivalent light on the left, plane-polarized light in the center, and cross-polarized
light on the right).
Figure 6. These views of thin sections from rocks bearing blue spinel (under plane-polarized light) show that blue
spinel is always associated with olivine (forsterite) and pargasite in calcite matrix. Clinochlore surrounds all main
minerals (left, field of view 1.5 mm). In the matrix, graphite and pyrrhothite are common accessory minerals
(right, field of view 0.5 mm). Cc = calcite, Clh = clinochlore, Gph = graphite, Ol = olivine, Pg = pargasite, Pyr =
pyrrhotite, Sp = spinel. Photomicrographs by Boris Chauviré.
Pg
Gph
Pyr
OI
Pg
CIh
Cc
Sp
scope or EDS with SEM. These included titanite, ru-
tile, zircon, graphite, apatite, several sulfide minerals
(again, see figure 6), and phyllosilicates. The sulfides
were mainly pyrrhotite (Fe1–xS; monoclinic) with
pentlandite exsolution lamellae ((Fe,Ni)9S8; cubic)
and violarite (FeNi2S4; cubic). Raman spectroscopy
helped to distinguish between different phyllosili-
cates, mainly clinochlore and phlogopite. Humite
was not observed in the marble, although this min-
eral is associated with red or purple spinel, as well as
ruby (Hauzenberger et al., 2003; Garnier et al, 2008).
Texture. The marble that hosts blue spinel has a gra-
noblastic texture, with millimeter to centimeter
grain size. SEM imaging with a backscattered elec-
tron detector showed exsolution features between
calcite and dolomite, and intergrown apatite and cal-
cite (figure 7a). Pentlandite lamellae in pyrrhotite
present two different morphologies. The first con-
sists of parallel flat lamellae less than 500 nm thick,
crossing some pyrrhotite crystals from end to end.
The second is lens-shaped, more than 1 µm thick and
about 3 µm long, often associated with parallel flat
lamellae (figure 7b).
Paragenesis. Blue spinel is observed only in olivine-rich
lenses, associated with dolomite and calcite (figure 8).
No blue spinel is observed in the marble when olivine
is absent. The spinel-rich lenses are elongated nearly
parallel to the regional foliation: roughly 45° toward
the northeast. Spinel and pargasite show inclusions of
apatite and sulfides similar in shape and composition
for both host minerals. This suggests that apatite and
sulfides preexisted spinel and pargasite. In some titan-
ite crystals, SEM imaging revealed inclusions of zircon
and pargasite. Therefore, titanite probably represents a
later stage of mineralization. Clinochlore crystals sur-
round all the other minerals (figure 7c), meaning it
probably crystallized later during a hydration phase,
and possibly during exhumation.
CHEMICAL COMPOSITION
Spinel. The composition of the three types of spinel
crystals was measured in thin sections using EDS,
and all rough samples were analyzed by EDXRF.
These analyses identified them as spinel sensu
stricto (MgAl2O4). Table 2 presents LA-ICP-MS
chemical analyses on representative samples of the
10 BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015
Figure 7. An inclusion of apatite in pargasite exhibits intergrowth with calcite (slightly magnesian; left, magnifica-
tion 750×). In most cases, pyrrhotite inclusions have exsolutions of pentlandite, a sulfide with higher nickel con-
tent (center, magnified 1200×), which also contains cobalt. Using scanning electron microscopy with
backscattered electron imaging, sensitive to the atomic number, a petrographic thin section of marble-bearing
blue spinel shows that spinel and olivine are surrounded by clinochlore. The marble is composed of calcite and
dolomite (right, magnified 65×). Ap = apatite, Cc = calcite, Clh = clinochlore, Dol = dolomite, Ol = olivine, Pen =
pentlandite, Pg = pargasite, Pyr = pyrrhotite, Sp = spinel.
A CB
Dol Clh
OI
Sp
Cc
Pg
Sp
Pen
Pyr
Ap
Cc
TABLE 2. Trace-element composition of three spinel
samples, measured by LA-ICP-MS.
Element
(ppma)
Li
Be
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Detection
Limit (ppma)
4
10
2
1
3
5
20
0.2
1
1
4
0.2
SKYBLU1
2778
552
3
11
16
238
11,009
84
85
4
7242
299
GREBLU2
6030
946
3
6
8
106
9362
22
29
4
4887
1088
SATBLUchem
2120
32
202
362
1111
287
12,794
1236
2514
4
1047
234
three color categories. The main impurities detected
were Li, Fe, and Zn. Significant traces of Be, Ti, V, Cr,
Mn, Ga, Ni, and Co were also detected. All analyzed
spinels had concentrations of Ga, Zn, and Li, consis-
tent with those observed only in natural blue spinel
(Muhlmeister et al., 1993; Krzemnicki, 2008; Sae-
seaw et al., 2009). All the samples presented nearly
uniform concentration in iron and in copper: around
10,000 ppma (equal to 1 atomic percent) and 4 ppma,
respectively. The other elements showed strong vari-
ation among samples. Sample SATBLUchem (satu-
rated blue) was enriched in Ti, V, Cr, Mn, Co, and Ni
compared to the other samples. Samples SKYBLU1
and GREBLU2 were enriched in Be and Zn compared
to SATBLUchem. GREBLU2 is also enriched in Li
and Ga compared to the two others.
Host Rocks. The chemical composition and charac-
teristics of associated minerals were also examined
with EDS analysis. Olivine is 99% pure forsterite
(Mg2SiO4). Pargasite is rich in titanium, sodium, and
chlorine. Apatites are fluorapatites with up to 20%
chlorine in substitution of fluorine. In two thin sec-
tions, we analyzed one REE-rich unknown mineral
and several molybdenum- and tungsten-rich un-
known minerals. Cobalt was found in sulfides, as high
as 1.5 wt.% in pentlandite and 3.5 wt.% in violarite.
SPECTROSCOPIC PROPERTIES OF BLUE SPINEL
UV-Vis Absorption Spectra. All UV-visible spectra
showed a broad, intense absorption band between 500
and 670 nm composed of several narrower bands at
about 545, 550, 560, 580, 590, and 625 nm (figure 9).
Two transmission windows were seen in the visible
part of the spectra, in the violet to blue region (400–
500 nm) and in the red region (670–700 nm). We also
observed several weak peaks between 300 and 500 nm
at about 371, 386, 418, 427, 455, 460, and 480 nm. The
bands at 427 and 460 nm are not visible on the spectra
that show the most intense main band between 500
and 670 nm (samples SKYBLU2 and SATBLU1). Ad-
ditionally, we noted a large, weak band centered at
about 440 nm only on the SKYBLU samples. For sam-
ples GREBLU1 and SATBLU1, we also note an in-
creasing absorption from 450 nm toward the UV.
Raman and Luminescence. The Raman spectra were
typical of spinel, with weak peaks at 405, 665, and
766 cm–1 (figure 10a; Fraas et al., 1973). The 405 cm–1
peak was 9 cm–1 wide, evidence that the analyzed
spinels were natural and unheated (Krzemnicki,
2008; Saeseaw et al., 2009). However, this Raman sig-
nal of spinel was overwhelmed by luminescence
with the two available excitation wavelengths (514
or 647 nm). The luminescence band was centered at
107 cm–1 for the 647 nm excitation wavelength, cor-
responding to a 650 nm emission (figure 10b). In this
case, the sample showed a strong red luminescence
(figure 10b, inset) consistent with a broad band emis-
sion centered at 650 nm. In addition, many weak
peaks between 673 and 710 nm, grouped in apparent
triplets, were visible: 685, 687, and 689 nm; 696, 697,
and 700 nm; and 704, 707, and 709 nm (figure 10b).
DISCUSSION
Primary Geological Origin.Red and blue spinels are al-
ways found in marble (figure 11). Garnier et al. (2008)
proposed that this marble originated from an old car-
BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015 11
Figure 8. In Vietnam, primary blue spinel deposits appear as approximately lens-shaped bodies rich in olivine.
These lenses are hosted in marble, and pargasite is found throughout the surrounding marble. Photo and drawing
by Boris Chauviré.
10 cm
Marble Pargasite Blue Spinel Olivine
5 mm
bonate platform (considered Precambrian to Permo-Tri-
assic), which later metamorphosed. Graphite crystals
in these Vietnamese marbles likely derive from
metamorphism of organic matter (Giuliani et al.,
2003; Garnier et al., 2008). As already mentioned,
blue spinels are always associated with olivine
(nearly pure forsterite). This paragenesis is typical of
the granulitic metamorphic facies (high temperature
above 550°C for a CO2-rich system; Bucher and Frey,
1994; Janardhan et al., 2001; Proyer et al., 2008). Par-
gasite is ubiquitous in marble, also representing a
high-temperature phase. Pargasite, olivine, and spinel
are nearly contemporaneous, and they may have
crystallized from the destabilization of diopside with
increasing pressure and temperature in a prograde re-
action (Proyer et al., 2008; Ferry et al., 2011).
We detected some fluorine and chlorine in apatite
and pargasite, and some sodium, lithium, and beryl-
12 BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015
Figure 9. The UV-Visible spectra of typical blue
spinels from Vietnam show a major composite ab-
sorption band between 500 and 650 nm, a large trans-
mission window in the blue to violet region, and a
smaller one in the red. The bands at 371, 386, 418,
455, 460, 480, 560, and 590 nm are due to Fe2+.The
bands at 545, 550, 580, and 625 nm are due to Co2+.
The band at 427 nm is not allocated.
UV-V
ISIBLE
S
PECTRA
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
WAVELENGTH (nm)
ABSORPTION COEFFICIENT (cm
–1
)
0
0.5
1.0
1.5
2.0
2.5
0
350 400 450 500 550 600 650 700
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
SATBLU1
SKYBLU 2
GREBLU1
Band due to Co
2+
Band due to Fe
2+
Band not allocated
Figure 10. The samples in this study displayed the
typical Raman signal for spinel (top), with a strong
continuum due to cobalt luminescence. But when the
concentration in cobalt was too high, luminescence
overwhelmed the signal for spinel. In some samples,
spectra acquired using a 647 nm excitation showed
additional luminescence peaks of chromium (the so-
called organ pipe spectrum, bottom). Under green
laser excitation (514 nm), the sample reacted with a
red luminescence (bottom photo).
R
AMAN
S
PECTRA
100 200 300 400 500 600 700 800 900 1000 1100
RAMAN INTENSITY (x1000 COUNTS)
0.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
20.0
22.5
25.0
30.0
405
50 150 250 350 450 550 650 750 850 950
1050
115012501350 1450
0.0
5.0
10.0
15.0
20.0
25.0
30.0
40.0
RAMAN SHIFT (cm
–1
)
Co
2+
: 6 50 nm
673 nm
675 nm
685 nm
687 nm
689 nm
696 nm
697 nm
700 n m
709 nm
707 nm
704 n m
Cr
3+
GREBLU2
SATBLU1
665
766
35.0
27.5
lium in blue spinel. These elements are indications
that evaporitic rocks played a role during metamor-
phism (Proyer et al., 2008). Giuliani et al. (1993) and
Garnier et al. (2005, 2008) also proposed this hypoth-
esis from the study of fluid inclusions in gem ruby
from the Luc Yen area.
Different areas yielding ruby and red or blue spinel
show distinct characteristics. Red spinel and rubies
have a very similar paragenesis. Forsterite is only as-
sociated with blue spinel, and clinohumite is only as-
sociated with red spinel. Clinohumite can also grow
from diopside in a prograde reaction with dolomite
and water (Proyer et al., 2008). Ruby-bearing rocks are
very different from those containing blue spinel, as
they underwent different metamorphic histories. Be-
cause of the intense tectonic activity in Luc Yen, it is
possible that two rocks with very different geological
histories have been brought in contact.
Garnier et al. (2008) did not observe evidence of a
fluid circulating through the marble. They proposed
that aluminum and chromium originally sedimented
within the carbonate platform. These elements were
mobilized due to the presence of halogen elements
(fluorine and chlorine) from evaporitic rocks. We pro-
pose that the mobilization of Ni and Co happened
through the same process. Another hypothesis is that
Ni and Co were mobilized from amphibolitic rocks
interlayered in the marble (observed by Garnier et al.,
2006) via halogen-rich fluids. Fluids can be formed
by the metamorphism of clay minerals, evaporate,
and organic matter (Giuliani et al., 2003; Garnier et
al., 2008).
Proposed Geological History. The ancient Paleo-
Tethys Ocean (possibly Proterozoic to Permo-Trias-
sic) separated the China (Yangtze) and Indochina
cratons (now Vietnam, Laos, Cambodia, Thailand,
and Myanmar). In this ocean, a carbonaceous plat-
form developed by sedimentation. Tectonic move-
ments caused the closing of this ocean, and
evaporitic minerals were deposited. The two main
blocks (Yangtze and Indochina) subsequently col-
lided, and all the sedimentary and magmatic rocks
of the oceanic crust underwent intense deformation
and metamorphism. During the collision, the meta-
morphism of the mix of former carbonate platform
minerals and some detritic material (such as clays)
deposited with it may have led to the formation of
diopside through the following reactions:
tremolite + calcite →diopside + dolomite
dolomite + quartz →diopside + carbon dioxide
By increasing metamorphism, diopside destabi-
lized into olivine, spinel, and clinohumite. The re-
duction of evaporitic minerals such as sulfates
formed chlorine- and fluorine-rich fluids. These flu-
ids were involved in the mobilization of aluminum
and other elements such as chromium (Giuliani et
al., 2003; Garnier et al., 2008).
Some processes remain poorly understood. Why do
some areas show clinohumite with red spinel while
others show olivine and blue spinel? What is the main
difference responsible for mobilizing more chromium
(red spinel) or more cobalt (blue spinel) in the marble?
Origin of Blue Color and Color Change in Viet-
namese Blue Spinels. The main absorption band be-
tween 500 and 670 nm, the dominant origin of color
in these blue spinels, is composed of a series of bands
at approximately 545, 550, 560, 580, 590, and 625 nm
BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015 13
Figure 11. Blue spinel from Luc Yen in its marble host.
Photo by Vincent Pardieu/GIA.
(again, see figure 9). Bands at 545, 550, 580, and 625
nm are due to cobalt (Co2+) substituting for Mg2+ in
tetrahedral sites of the spinel structure (Wherry,
1929; Pappalardo et al., 1961; Shigley and Stockton,
1984; Kuleshov et al., 1993; Muhlmeister et al., 1993;
Delaunay et al., 2008; Duan et al., 2012; Bosi et al.,
2012; D’Ippolito et al., 2015). The remaining absorp-
tion bands (at 371, 386, 418, 455, 460, 480, 560, and
590 nm) are allocated to iron (Fe2+) in tetrahedral sites
of the spinel structure (Gaffney, 1973; Dickson and
Smith, 1976; Muhlmeister et al., 1993; Delaunay et
al., 2008; D’Ippolito et al., 2015). A weak band ob-
served at 427 nm is not attributed but may be linked
with other measurable elements such as Ni. Conse-
quently, the spectra show transmission windows be-
tween 300 and 500 nm and between 700 and 900 nm
that explain the blue color. As expected, the spectra
show that iron (Fe2+) and cobalt (Co2+) are the main
chromophore elements. The other trace elements de-
tected either do not give rise to absorption in the vis-
ible range or are much less efficient absorbers than
cobalt. Chromium, which is the main chromophore
for red and pink spinel, makes a significant contribu-
tion to color if the concentration is above 1000 ppma.
(Muhlmeister et al., 1993; T. Häger, pers. comm.,
2014). Cr concentration in SATBLU samples is bor-
derline, but the Co concentration is higher, too. The
contribution is considered negligible.
We observed that the SATBLU samples, which had
the most saturated color, also had the most important
cobalt optical absorption. In addition, the main band
had an absorption coefficient greater than 10 cm–1, and
the iron optical absorptions observed were weak. For
the parcel classified as SKYBLU, iron and cobalt opti-
cal absorption seemed to have a similar importance in
the optical spectra, reaching a maximum of 2 cm–1 as
compared to SATBLU samples. GREBLU samples had
the most significant iron band, but the main absorp-
tion band only reached 0.5 cm–1. We also observed a
correlation between the cobalt absorption bands and
the color saturation.
SATBLU2 had a Fe/Co value of approximately 10
(table 2), and the SATBLU samples had the most sat-
urated color (table 1). GREBLUE2, with a Fe/Co ratio
of about 425 (table 2), had a visible gray hue compo-
nent (table 1). For intermediate Fe/Co ratios of about
130 (measured on SKYBLU1; see table 2), the spinel
had a sky-blue color (table 1). Moreover, sample
GREBLU1, which had the grayest color, showed
more significant bands due to Fe2+. We propose that
the GREBLU samples are colored mainly by iron and
the SATBLU samples by cobalt. The SKYBLU sam-
ples’ colors arise from both iron and cobalt absorp-
tion. Hue differences are more significantly
controlled by iron (with different species), while sat-
uration is largely dictated by cobalt (D’Ippolito et al.,
2015).
Using chemical and spectroscopic analysis from
eight of our samples, we calculated the molar absorp-
tivity of cobalt in spinel (sensu stricto) for three ab-
sorption bands. At wavelengths of 545, 580, and 625
nm, we took the apparent maximum of each band. We
14 BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015
Figure 12. A gem mer-
chant examines a blue
spinel from the Luc
Yen district. Photo by
Vincent Pardieu/GIA.
determined a molar absorptivity of 530±29, 664 ±18,
and 586±11 L·mol–1·cm–1, respectively. For the sake of
comparison, we calculated the molar absorptivity of
iron in spinel (sensu stricto) for bands 371, 386, 480,
and 590 nm. These bands are attributed to ferrous iron
(Fe2+) in the tetrahedral site. Our chemical analysis
measured only the total iron content. Assuming all
iron was in the form Fe2+, we propose that the molar
absorptivity of ferrous iron in the tetrahedral site had
an order of magnitude of about 30 L·mol–1··cm–1 for
each band. With this method, values of molar absorp-
tivity are not very accurate but provide a working as-
sumption for our preliminary study. We recognize
that further investigation is needed to fully under-
stand the color in blue spinel. A Gaussian decompo-
sition of spectra can improve the precision of these
values. Nevertheless, it is apparent that in spinel,
Co2+ is approximately 20 times more efficient at ab-
sorbing light, and thus creating color, than Fe2+ (con-
sistent with D’Ippolito et al., 2015).
Spectra have two transmission windows between
350 and 500 nm (in the blue region) and between 670
and 900 nm (in the red region). This explains the pink
to red reaction under the Chelsea filter and the color
change. Indeed, the Chelsea filter probes a transmis-
sion window in the red. The color change is also ex-
plained when the spectral composition of the
lighting environment is compared with the absorp-
tion spectra of spinel, although this change is not ob-
served in every example. Compact fluorescent light
emits more in the blue region than in the red, and
therefore the spinel appears blue. Under incandes-
cent light, which is richer in red, spinel displays a vi-
oletish blue color that is mostly blue with minor red.
Origin of Red Luminescence. Under laser excitation,
our samples showed a strong red luminescence. In
spectra acquired using a 647 nm excitation, we ob-
served several peaks (in groups of three) between 673
and 710 nm (at about 673, 675, 685, 687, 689, 696,
697, 700, 704, 707, and 709 nm; see figure 10b). These
peaks are known to be due to trivalent chromium
(Cr3+) substituting for aluminum in the octahedral
site (Burns et al., 1965; Wood et al., 1968; Skvortsova
et al., 2011). The broad band centered at 650 nm is
allocated to divalent cobalt in the tetrahedral site of
the spinel structure (Abritta and Blak, 1991;
Kuleshov et al., 1993). These luminescence behaviors
are consistent with our chemical analysis, as the
strongest luminescence was observed in the SATBLU
samples, which had higher concentrations of Cr3+
(1111 ppma) and Co2+ (1236 ppma).
CONCLUSION
We confirmed that the saturated “cobalt-blue” color
of Vietnamese spinels (figures 12 and 13) is due pre-
dominantly to Co2+ substituting for Mg2+ in the tetra-
hedral site of the spinel structure. For the most
saturated blue spinel, cobalt is the main coloring
agent, even if iron is more abundant. Indeed, cobalt
is a powerful coloring agent, with a molar absorptiv-
ity between 500 and 700 L·mol–1·cm–1 depending on
wavelength, whereas iron (Fe2+ in the tetrahedral site)
has a molar absorptivity of about 30 L·mol–1·cm–1.
The higher the iron/cobalt ratio is, the grayer the
color. The red transmission window of these gems
explains both their pink to red Chelsea filter reaction
and their slight change of color from blue to “laven-
der” with a change of lighting environment. The red
luminescence is due to both Cr3+ and Co2+, and it may
have a minor influence on the perceived color.
This study offers clues to the definition of
“cobalt-blue” spinel. Cobalt is actually the main
chromophore, but the presence of iron is also signif-
icant. The term “cobalt-blue” can be clarified by fur-
ther investigations on the significance of each
chromophore elements (iron and cobalt). These in-
vestigations can propose a limit on the ratio of
iron/cobalt above which the term “cobalt-blue” can-
not be used.
Spinels from Luc Yen contain few inclusions.
Fractures and fingerprints were the most common
inclusions found. Sometimes, we observed parallel
elongated tubes with black, irregular solid inclusions
associated.
BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015 15
Figure 13. Vietnam’s spinel production yielded this
2.59 ct cobalt blue gem. Photo by Robert
Weldon/GIA, courtesy of Palagems.com.
From a geological standpoint, gem-quality blue
spinels are associated with intense metamorphism.
Their marble host results from the metamorphism
of an ancient carbonaceous platform. This platform
was located in the Paleo-Tethys Ocean, which sepa-
rated Indochina and China. During the convergence
of these “paleo-continents,” the ocean closed off, ac-
companied by the formation of evaporitic rocks. The
ocean crust, associated with the carbonaceous plat-
form and evaporitic platform, was sandwiched be-
tween the two continents. The collision led to the
metamorphism of the evaporite rocks, in turn pro-
ducing fluids mobilizing some elements, possibly in-
cluding cobalt. Spinel grew in the marble during this
intense metamorphism. These processes of meta-
morphism and fluid interaction led to the crystalliza-
tion of attractive blue spinels in the marble
mountains of Luc Yen.
16 BLUE SPINEL FROM LUC YEN, VIETNAM GEMS & GEMOLOGY SPRING 2015
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REFERENCES
ABOUT THE AUTHORS
Mr. Chauviré (boris.chauvire@univ-nantes.fr) is a PhD student at
the Laboratoire de Planétologie et Géodynamique de Nantes,
France, and Dr. Rondeau is an assistant professor at the same
laboratory (CNRS Team 6112). Dr. Fritsch (CNRS Team 6502) is a
professor of physics at the University of Nantes, Institut des
Matériaux Jean Rouxel. Mr. Ressigeac is product manager for
Montepuez Ruby Mining, Mozambique. Mr. Devidal is an engineer
specialist of ICP-MS-LA at the Laboratoire Magmas et Volcans,
Clermont Ferrand, France.
ACKNOWLEDGMENTS
We are grateful to Vincent Pardieu, senior manager of field gemol-
ogy at GIA’s Bangkok laboratory, for his valuable aid during the
preparation and progress of the expeditions. We thank Vincent’s
contacts for acquiring blue spinel samples. We are grateful to Mr.
Chuãn, our guide, for his knowledge of the field and his logistical
support. We also thank Pham Van Long, director of the Center for
Gem and Gold Research and Identification in Hanoi, for his logisti-
cal support and for exporting the samples collected. GIA’s labora-
tory in Bangkok and its director, Kenneth Scarratt, provided
technical and logistical support. Jean-Pierre Lorand (LPGN-
CNRS) generously shared his knowledge about sulfides. We
thank Alexandre Droux from the Laboratoire Français de Gem-
mologie for EDXRF measurements. We also thank Tobias Häger
of Johannes Gutenberg University in Mainz, Germany, for his help
with interpreting UV-Vis spectra. We also thank reviewers that
participated to improve this study.
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