A closer look at Vietnamese spinel - Kingdom of Red Spinels

A closer look at Vietnamese spinel
By Christopher P. Smith, C.R. “Cap” Beesley,
Elizabeth Quinn Darenius, and Wendi M. Mayerson
American Gemological Laboratories
Over the past couple of years, a supply of attractive
spinel exhibiting various colors has been entering the
international gemstone market from a relatively small
mining region in Northern Vietnam. The majority of these gems
are being processed and marketed through Bangkok, Thailand,
and finding their way to the major American, European and
Asian centers.
In recent months, Vietnam has produced a significant quantity of fine quality spinels,
ranging in color from red to pink and violet to blue. Photograph by Fred Kahn and Sun Joo
Chung.
G E M O L O G Y
Spinel exhibiting a broad spectrum of colors is known to
derive from many countries around the world, however, the
two most well-known and historical deposits are found in Sri
Lanka and in the area surrounding the Mogok valley in Burma
(Myanmar). More recent finds of spinel in places such as Namya
Zeik (northern Burma), Kenya, Tajikistan and Tanzania have led
to an increased supply of this highly desirable and sought after
gem material.
Spinel from the Luc Yen region of northern Vietnam began
making its way into the market as mineral specimens and
faceted gemstones during the late 1980s and the early 1990s.
This coincided with the discovery
of ruby deposits in Luc Yen and
Quy Chau (Kane et al, 1991). More
recently, Vietnam has re-emerged
and made an exciting re-entrance
into the world of fine gem spinel
with its latest production. The
main part of the most recent
production yields faceted stones
that range from below 1 carat to
1-5 carat sizes. More rarely, some
fine spinels have been produced
in sizes larger than 10 carats, and
even up to 20 carats in very fine
quality (M. Hatik pers. comm.
2008).
Color appearance
The most famous and popular
color of spinel is red, although
it actually occurs in a wide range
of hues and saturations. The spinel
from Vietnam is no exception,
as it exhibits a number of very
attractive colors and shades. The
majority of Vietnamese spinel
occurs in a deep red to purplish
red hue. Additionally, a number
of lively, attractive pink colors
are also produced. Of the more
exceptional colors, Vietnam has
also produced a number of very
attractive violet to violet-blue
stones, as well as a small number
of “cobalt-blue” spinel, which are
highly sought after and prized by
collectors.
Gemological properties
The standard gemological
properties for a number of
Vietnamese spinels examined
InColor l Spring 2008 31

G E M O L O G Y
by the authors are consistent with those reported in the
gemological literature for natural spinel. The stones tested
were all found to be singly refractive, with refractive index
readings ranging from 1.712 to 1.718. Their specific gravity
was determined hydrostatically to range from 3.54 to 3.71.
The fluorescent reaction to both long-wave and short-wave
UV radiation was strong red (LWUV) and weak to moderate
red (SWUV) for the red group. The pink group exhibited a very
strong red (LWUV) and moderate to strong orangy red (SWUV)
reaction, whereas the violet-to-blue stones were inert to both
LWUV and SWUV excitation.
When examined with a Chelsea filter over a strong transmitted
light source, the “cobalt-blue” Vietnamese spinel exhibited a
strong red reaction, while the violet-to-blue group exhibited a
weak red or pink reaction.
Absorption spectroscopy
The absorption spectra of the spinel from Vietnam were
tested using a standard desk-model spectroscope and
a UV/Vis/NIR spectrophotometer (range 850-280 nm). The
red-to-pink group possessed two dominant broad bands
centered at approximately 540 and 388 nanometers (nm),
with a shoulder positioned at approximately 415 nm, as well as
a series of narrow, sharp peaks at approximately 694, 685, 665
and 640 nm. These absorptions are primarily due to chromium
(Cr3+ ) in octahedral coordination (see for example Fritsch and
32 InColor l Spring 2008
Vietnam has also
produced a very small
supply of “cobalt-blue”
spinel, such as this very
fine example. Photograph
by Fred Kahn and Sun Joo
Chung.
Violet: Fe 2+ >Co 2+ Violet-Blue: Fe 2+ /Co 2+ “Cobalt Blue”: Fe 2+ Co 2+ , whereas in the “cobalt-blue” colored sample
the ratio was Co2+ >Fe2+ . Intermediate colors exhibited
intermediate ratios.
Rossmann, 1988).
All of the spinels in the violet-to-blue group showed
a combination of spectral bands related to cobalt (Co 2+
in tetrahedral coordination) and iron (Fe 2+ in tetrahedral
coordination). The Co 2+ -related absorptions consisted of
a series of three superimposed broad bands positioned
at approximately 625, 585 and 550 nm. The Fe 2+ -related
absorptions consisted of a larger series of bands positioned
at approximately 555, 477, 455, 385 and 370 nm. In the violet
spinels, the relative ratio of these absorptions is Fe 2+ >Co 2+ ,
with decreasing Fe 2+ absorption and increasing Co 2+ occurring
as the color trended towards a more blue color. No chromium
related absorptions were recorded in the violet-to-blue group
examined in this study.
Vietnamese spinels play host to a variety of foreign minerals.
These range from transparent colorless crystals, to black
platy inclusions. Photomicrograph Christopher P. Smith, 88x.
A distinctive inclusion feature in the violet-to-blue series
of Vietnamese spinel consists of planar concentrations of
parallel needles. These needles may be confined to small
“patches” or they may extend the length of the interior.
Photomicrograph Christopher P. Smith, 48x.

G E M O L O G Y
Often, Often confined “patches” of these parallel needles can be
seen surrounding a small mineral inclusion. Photomicrograph
Christopher P. Smith, 60x.
These planar concentrations of parallel needles also
occurred intersecting other groups or oriented individually
in different directions. Photomicrograph Christopher P. Smith,
112x.
Healed fractures composed of aligned octahedra of various
sizes are commonly present. Photomicrograph Christopher P.
Smith, 58x.
34 InColor l Spring 2008
Chemical composition
The samples were tested using energy dispersive X-ray
fluorescence spectroscopy to determine their major to
minor and trace element composition. Besides aluminum (Al)
and magnesium (Mg), which are intrinsic to spinel (MgAl O ), 2 4
the red-to-pink spinels from Vietnam also contained iron (Fe),
along with chromium (Cr) as major to minor elements. Other
minor to trace elements included zinc (Zn), vanadium (V),
titanium (Ti) and gallium (Ga).
The violet-to-blue group contained iron (Fe) and zinc (Zn)
as major to minor elements, with only trace concentrations
of manganese (Mn) and gallium (Ga). Cobalt (Co) was not
detected in the violet-to-blue group stones using this method
of chemical analysis. It is presumed that cobalt occurred below
the detection limit of the spectrometer. These compositions
are consistent with that of natural spinel from several other
deposits (see for example Muhlmeister et. al, 1993).
Inclusion features
The red-to-pink spinels from Vietnam display a varied
array of internal features. These consist of planar growth
structures that follow octahedral crystal faces, healed fractures
of various patterns, and a variety of foreign mineral inclusions.
The violet-to-blue group possesses a more distinctive series of
internal features. These consist primarily of arranged, planar
concentrations of parallel needles which are either isolated in
rather small “patches” or are longer and extend throughout
the stone. On occasion, these small “patches” of needles are
seen surrounding a tiny mineral inclusion or intersecting one
another from multiple directions. Lines or stringers of pinpoint
particles are also observed, as well as variously sized octahedra
aligned along healed fracture planes.
Conclusion
Although Vietnamese spinel has been available for more than
a decade, recently a significant supply of fine, gem-quality
faceted stones has been entering the market from the Luc Yen
mining region in northern Vietnam. These gems exhibit a wide
range of colors and shades, from red to pink, as well as rarer
colors that range from violet to blue. For the spinel enthusiast
and collector, this production offers an opportunity not only to
collect fine, gem-quality specimens from an unusual source, but
to collect samples in a rare and appealing range of colors.
References
Fritsch E., Rossmann G.R. (1988) An update on color in gems.
Part 3: Colors caused by band gaps and physical phenomena.
Gems & Gemology, Gemology Vol. 24, No. 2, pp. 81-102.
Kane R.E., McClure S.F., Kammerling R.C., Khoa N.D., Mora
C., Repetto S., Khai N.D., Koivula J.I. (1991) Rubies and fancy
sapphires from Vietnam. Gems & Gemology, Gemology Vol. 27, No. 3, pp.
136-155.
Muhlmeister S., Koivula J.I., Kammerling R.C., Smith C.P.,
Fritsch E., Shigley J.E. (1993) Flux-grown synthetic red and blue
spinels from Russia. Gems & Gemology, Gemology Vol. 29, No. 2, pp. 81-98.
Acknowledgements: The authors would like to thank the
following people for providing the samples tested during this
study: Sam Khafi, Mourad Hatik and Bear Williams.