Garnet: From Stone to Star
Six-star garnet from
Emerald Creek, Latah
County, Idaho, USA.
PHOTO SOURCE : WWW.
STARGEMSTONES .COM
Laurence Galoisy*
1811-5209/13/0009-453$2.50
G
DOI: 10.2113/gselements.9.6.453
arnet often occurs as naturally multifaceted, brightly colored, transparent, single crystals. These crystals represent chemically diverse
solid solutions with a remarkable range of colors, which are largely
controlled by the crystal chemistry of transition elements such as Fe, Mn,
Ti, Cr, and V. These same optical properties have given garnet important
cultural and historical relevance as a sought-after gemstone, from biblical
times to the present day.
Almandine
( Fe 3 A l 2 Si 3 O 1 2 )
was named by Pliny the Elder
(23–79 AD) and comes from
alabandicus, in reference to a
stone found in Alabanda (Anatolia,
Turkey). The name spessartine (Mn3Al 2 Si3O12 ) comes from
Spessart in Bavaria (Germany),
where the mineral was discovered around 1800 AD. Grossular
KEYWORDS : garnet colors, structure–color relationships,
(Ca 3Al 2 Si 3 O12 ) is derived from
UV–visible–NIR absorption spectroscopy, garnet and history
the Latin grossularia (the botanical name for gooseberry), and
the species was named for green
crystals found in Siberia. Finally,
A STONE… A STAR
pyrope (Mg3Al2 Si3O12 ) was named from the Greek pyropos
A key rock-forming mineral in the Earth’s crust and mantle,
(pyr = fi re, ops = eye).
garnet is present in a diverse range of geologic settings and
geographical localities, resulting in remarkable variations
Two figures of the 19th century gave their names to garnet
in crystal color and size (see Baxter et al. 2013 this issue).
minerals. Andradite (now defi ned as Ca3Fe2 Si3O12 ) was
Garnet has attracted attention as a gemstone since the
named after José Bonifacio de Andrade e Silva, a Brazilian
Bronze Age due to the beauty of its large euhedral crystals.
statesman, naturalist, professor, and poet (1763–1838).
The relative hardness of garnet (6.5 to 7 on Moh’s hardness
Uvarovite (Ca3Cr2 Si3O12 ) was named after Count Sergey
scale) makes it a durable gemstone that can withstand the Uvarov (1786–1855), a Russian mineral collector (Whittaker
trials of time, and garnet has been used to trace ancient 1984).
trading routes and reconstruct the origins of individual
crystals or gemstone populations (Calligaro et al. 2002).
Garnet and the Cloisonné Art:
The variation in crystal color encompasses white, green,
Garnet at the Beginning of Our Era
orange, red, and a rare blue color, and some garnet crystals
During the early Middle Ages (ca 300 –900), the
even exhibit an internal “star” pattern. This variation is
cloisonné technique utilized inlays of gemstones to
related to the presence of transition elements such as Fe, decorate metalwork objects. The decoration consisted of
Mn, Ti, Cr, and V and to the optical effects of certain fi ne dividing walls (cloisons) used to separate the object of
mineral inclusions. Spectroscopic techniques demonstrate interest (typically jewelry) into different compartments,
how the nature and concentration of these cations explain each inlaid with a stone (FIG. 1A). The gemstones used were
the color variation. This article reviews some aspects of rhodolite and pyrope garnets, and the technique consisted
the cultural heritage of garnet and discusses the crystalof fi xing thin sections of red and translucent crystals into
chemical factors that produce the myriad colors of this
gold-separated cells. Cloisonnés were produced by various
fascinating mineral supergroup.
tribes from Scandinavia and Asia (Arrhenius 1985) and
GARNET IN HISTORY
Origins of Garnet Names
Garnet has been valued for millenia. In his treatise on
stones, Theophrastus (372–287 BC) called it “anthrax,”
meaning coal or carbuncle, by analogy to the red color of
glowing coal. The name garnet was coined by German
theologian and philosopher Albrecht von Bollstädt (1193–
1280) from the Latin word granatus, in reference to the red
seeds in a pomegranate, which resemble the shape and
color of the mineral.
* IMPMC Université Pierre et Marie Curie
Place Jussieu, 75005 Paris, France
E-mail: [email protected]
E LEMENTS , V OL . 9,
PP.
453–456
were valued by high-ranking officials. For example, garnetbearing golden bees and fl ies were found in the sepulcher
of the fi rst monarch of France, King Childéric I, who was
buried near the church of Saint-Brice de Tournai (FIG. 1B).
Garnets from the 5th to 7th century were also found in
cloisonné jewels in the royal necropolis of the Saint-Denis
Basilica, the burial place of the kings of France, near Paris.
Bohemian Pyrope in the 16th –20th Centuries
During the 16th century, a rich jewelry industry utilizing
pyrope garnet from Turnov (Czech Republic) began in
Bohemia. The host rock for these crystals is serpentinized
peridotite, and these pyrope crystals are renowned for their
transparency and brightness (Fiala and Paděra 1977). In
1780, the great Venetian smith Callegari moved to Turnov,
attracting other craftsmen to the area, and approximately
a century later, during the Victorian Age, most of the
453
D ECEMBER 2013
A
B
C
(A) Detail of the cloisons separating the inlays of a
gold and cloisonné garnet strip from the Staffordshire
hoard. The garnet minerals are about 2 × 2 mm. (B) Gold bee and
fly inlaid with garnet found in the sepulcher of the first monarch of
France, King Childéric I.
(C) Pendant made with garnets from Perpignan. PHOTO SOURCES :
GALLICA .BNF.FR / BIBLIOTHÈQUE N ATIONAL DE FRANCE (A); ©B IRMINGHAM M USEUM
TRUST (UK) (B); AND PAULIGNAN JEWELRY, PERPIGNAN, P YRÉNÉES- ORIENTALES,
L ANGUEDOC-ROUSSILON, FRANCE (C)
gem-pyrope jewelry in the world was produced in Bohemia.
Franz Joseph I (1830–1916) gave a unique gold cross jewel
decorated with Bohemian garnets to his wife Elisabeth
(better known as “Sissi”). Following a decline in popularity
after World War II, there is currently a renewal of interest
in this superb gem, with Bohemia again producing many
fi ne examples (Schlüter and Weitschat 1991).
different garnets (e.g. Cr substitution in pyrope produces a
pink-purple color, whereas Cr in andradite produces a green
color). This phenomenon is discussed below, organized by
compositional group.
FIGURE 1
Garnets and Traditions
Diverse beliefs are attached to garnet in folklore, legends,
and traditions. In the Talmudic legend, “the only light
in Noah’s Ark was given by an enormous red garnet.” In
medieval times, garnets were thought to cure depression,
protect against bad dreams, and give strength. In 1892, in
Kashmir, the Hunzas fought the British army with bullets
made with red garnets, in the belief that they were deadlier
than lead due to their red color, reminiscent of blood.
In Spanish astrology, garnet once represented the sun. In
France, garnets are the symbol of the Languedoc-Roussillon
region (Fonquernie 2006). The traditional garnet jewelry
made near Perpignan, in the French Pyrénées, is particularly renowned (FIG. 1C ). Catalan jewelers now try to
preserve the method of stamping and the mounting of
garnets in bezel settings, as done during the 17th century.
In 1912, the American National Association of Jewelers
adopted garnet as the birthstone for January in the United
States, to mark a “rich and love-filled new year.” Nowadays,
garnet celebrates the second and sixth years of marriage.
In the United States, it is the state mineral of Connecticut
and it is New York State’s gemstone, due to the abundant
garnets from the Barton mine (Adirondack Mountains,
NY). In 1967, a star garnet was chosen as the gemstone of
Idaho (see inset photo).
GARNET: THE RAINBOW GEMSTONE
Natural garnets are rarely devoid of color (FIGS. 2, 3). The
pure colorless end-member of the pyrope family is rarely
found, contrary to the grossular family end-member
(FIG. 3B). Transition elements substituted in the X and Y
sites of garnet’s crystal structure provide a broad range of
coloration, including red, green, yellow, pink, and orange
(Rossman 2009). Some garnets exhibit an alexandrite
effect, i.e. their color changes with the lighting conditions
(Gübelin and Schmetzer 1982). Rare blue garnets with this
peculiar alexandrite effect have been reported from the
Bekily province of Madagascar. Crystal-chemical interactions of transition elements within the garnet structure are
primarily responsible for the colors we perceive. However,
the same transition element can produce different colors in
E LEMENTS
Color of Pyrope Garnets
Pyrope-rich garnets are widespread (FIG. 2A, C). Pyrope
garnets are red to brownish red, reflecting increasing Fe2+
content as Fe2+ substitutes for Mg2+ in the dodecahedral X
site. Usually, Fe2+ is found in octahedral sites in ferromagnesian minerals, imparting a green coloration in olivine
and orthopyroxene (Burns 1993). Cr3+ substituting for Al3+
in the octahedral Y site gives the garnet a pink-purple
tint, the intensity of which increases with Cr3+ content.
The ultraviolet–visible–near-infrared (UV–Vis–NIR) absorption spectrum of a red-purple pyrope (FIG. 4) shows the
absorption bands of Fe2+ and Cr3+ in the X and Y sites,
respectively (Juhin et al. 2008). In the dodecahedral X
site, Fe2+ shows electronic transitions in the near-infrared
range, from 5000 cm-1 to 10,000 cm-1, that do not affect
the visible domain. However, from 10,000 cm-1, absorption increases toward the UV region, a specific signature of
oxygen-to-metal (Fe2+) charge-transfer transition (OMCT).
This OMCT is responsible for the brown tint in pyrope–
almandine garnets.
Cr3+ in the octahedral Y site gives rise to two transitions
in the visible domain (FIG. 4). The coexistence of two
transmission windows (12,500–16,000 cm-1 and 19,000–
22,000 cm-1) causes the specific red-purple color of the
pyrope. Substitution of transition elements as impurities
into the structure of minerals always gives rise to important structural relaxation (Galoisy 1996). In Mg3Al2 Si3O12,
the Al–O distance is 1.89 Å, but the addition of Cr3+ ions
relaxes the Y site, expanding the Cr–O distance to 1.96 Å (as
in the pure end-member knorringite, Mg3Cr2 Si3O12 ). The
relaxation induces tilting of Z sites around Cr-substituted
Y sites and a deformation of the X site (Juhin et al. 2008).
With increasing Cr3+ content, the transmission window is
shifted towards the UV domain, with an accompanying
modification of crystal color from red to deep purple and to
green for knorringite (Carstens 1973). However, this color
change is attained without changing the Cr–O distance
from a value of 1.96 Å. The color modification is instead
related to a change in the effective charge of the oxygen
bound to Cr3+ (Juhin et al. 2008). Similar observations
are made for red Cr-spinels and rubies, with similar full
structural relaxation around substituted Cr3+ and a red
to green color change with increasing Cr concentration
(Juhin et al. 2007).
454
D ECEMBER 2013
A
B
C
(A) Raspberry rhodolite garnets with purple and pink
overtones, from Kagala, Tanzania. (B) Two reddish
brown crystals of almandine on matrix, from Fanny Gorge mine,
Spruce Pine, Mitchell County, North Carolina, USA.
(C) Bright golden and yellow as well as orange, pink, peach, rose,
cinnamon, and even color-change malaya garnets from Tanzania.
PHOTO SOURCES : JOHN PARISH (PHOTOS) AND L ARRY WOODS (CUTTING)
(WWW.JEWELSBYWOODS.COM) (A AND C); LOU PERLOFF (B)
Colors of Gems from the Pyrope Family
garnets come from a few sources in Tanzania, Idaho, India,
and Sri Lanka (see inset photo), and their color includes
dark to light purple and red. Asterism, also referred to
as a form of chatoyancy, is due to the scattering of light
normal to the direction of yellow rutile inclusions (Guinel
and Norton 2006).
FIGURE 2
Gemologists subdivide pyrope garnets into varieties such
as rhodolite and malaya, depending on color. Rhodolite
corresponds to the 50/50 member of the almandine–pyrope
solid solution. Its color is due to Fe2+ in the X site, together
with some Cr3+ substitution for Al3+ in the Y site, and
changes from pink to purple with increasing Cr3+ content
(FIG. 2A). Originally found in Cowee Valley (North Carolina,
USA) (Hidden and Pratt 1898), this garnet is also found in
Tanzania and is rare enough to be used as a gemstone.
Malaya is a light to dark pinkish orange, reddish orange, or
yellowish orange garnet that was discovered near the Umba
River in Tanzania and Kenya in the mid 1960s (FIG. 2 C).
The name malaya comes from the fact that it was typically
rejected by gems dealers in the early 1970s, malaia being
the Swahili word for outcast. However, with time, malaya
gradually became the reference for the pale orange-pink
gems of the pyrope–spessartite series, with color related to
the presence of both Fe2+ and Mn2+ in X sites.
Color of Almandine Garnets
The red-brown color of almandine (FIG. 2B) is modified
by the presence of octahedral Ti4+ and Fe3+ (Khomenko
et al. 2002). The rare garnets of the almandine–skiagite
(Fe32+ Fe23+ Si3O12 ) solid solution show a brownish yellow
tint due to an intervalence charge transfer (IVCT) between
Fe2+ and Fe3+ in edge-sharing X and Y sites, respectively
(Taran et al. 2007). Asterism in some pyrope–almandine
garnets is seen as stars with four or six radiating branches,
which change intensity with the viewing direction. These
A
B
F
G
Colors of Andradite and Grossular Garnets
Andradite is yellow-green (FIG. 3A) (Adamo et al. 2009),
becoming emerald green with substitution of Cr3+ into
the Y site (demantoid; Stockton and Manson 1983). Three
gem varieties belong to this family: (1) melanite (black),
used for mourning jewelry (FIG. 3D); (2) demantoid (green),
the name of which evokes similarities with diamond in
terms of brilliance and fi re; and (3) topazolite (yellow with
brown/black reflections), a rare variety containing Ti.
The exceptional refractive index of demantoid (1.885–
1.895) arises from the presence of Fe3+ (Liddicoat 1981).
Demantoid sometimes displays an outstanding golden
glow due to the presence of fibrous amphibole or chrysotile inclusions (“horsetail inclusions”). This feature helps
identify this garnet and gives the gem its high value. The
stone was discovered in 1898 in the Urals (Sissersk district,
Nizhny Tagil and Bobrovka River) and recently in Namibia
(1990) but without horsetail inclusions. The composition of
demantoid is remarkably constant at almost 97% andradite.
Colorless grossular is found at Sierra de las Cruces,
Coahuila, Mexico (FIG. 3B). Grossular gems usually display
a green or bright orange to orange-brown color (FIG. 3C), as
in the cinnamon type, hessonite. Green tsavorite (FIG. 3F),
C
D
E
Different types of garnet: (A) Andradite from Antetezambato, Diana,
Madagascar. (B) White grossular from Sierra de las Cruces, Coahuila,
Mexico. (C) Grossular from Jeffrey mine, Asbestos, Québec, Canada. (D) Melanite
from Thomas Range, Utah, USA. (E) Spessartine from Loliondo, Arusha region,
Tanzania. (F) Tsavorite from Merelani Hills, Lelatema Mountains, Arusha region,
Tanzania. (G) Uvarovite from Outokumpu, Northern Karelia, Finland. (A, B, C, D,
E, AND G) SAMPLES FROM THE MINERALOGICAL COLLECTION OF THE U NIVERSITÉ PIERRE ET MARIE
CURIE, PARIS, FRANCE, PROVIDED THROUGH THE COURTESY OF J.C. BOULLIARD. PHOTOS BY
A LAIN JEANNE-M ICHAUD, INSTITUT DE MINERALOGIE ET DE PHYSIQUE DES MILIEUX CONDENSÉS ;
(F) PHOTO COURTESY OF WWW.MARINMINERAL.COM
FIGURE 3
E LEMENTS
455
D ECEMBER 2013
Colors of Spessartine and Uvarovite Garnets
Spessartine has a bright orange color (FIG. 3E) and can
be confused with hessonite, the orange/green variety of
grossular. Mn2+, localized in X sites, imparts the orange
color (Moore and White 1972). Several deposits have
been discovered since 1991 (e.g. Namibia, Mozambique,
Nigeria, China, and Madagascar), and spessartine gems
are becoming more widely available. Often forming solid
solution with almandine and pyrope garnets, spessartine
is a valuable gem due to its high refractive index (1.81)
and hardness (7.5).
FIGURE 4
FROM J UHIN
ET
Uvarovite garnets show only a green coloration, which is
related to the presence of Cr3+ in Y sites (Amthauer 1976;
Andrut and Wildner 2001). Despite its outstanding color,
uvarovite is rarely found as gemstone as it is very scarce.
The best specimens (from Outokumpu, Finland) reach
10 mm but are opaque to translucent (FIG. 3G).
Ultraviolet–visible–near-infrared absorption spectrum
of a natural pyrope from Garnet Ridge, Arizona.
AL . (2008)
CONCLUSIONS
discovered in 1967 in northern Tanzania and later (2002)
in southern Madagascar (Feneyrol et al. 2013), contains
both Cr3+ (up to 1.2 wt%) and V3+ (up to 7.5 wt%). The
oxidation state and substitutional character of V have
recently been determined by spectroscopic and theoretical
calculations, showing a full structural relaxation around
V3+ in the substituted Y site (Bordage et al. 2010). Minor
Fe3+ gives Madagascar tsavorite a specific brown to yellow
coloration. Tsavorite is a precious gem, and single crystals
rarely exceed two carats.
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E LEMENTS
Garnets are widespread gemstones and have been much
loved throughout history. They are currently experiencing
a strong revival despite the fact that only a few locations
yield valuable stones. Nondestructive, spectroscopic
analyses are important for relating garnet’s chemistry to
its color properties, but much remains to be done to understand this structure–property relationship. The crystal
chemistry of garnets may also shed light on their formation conditions. But further studies are needed to fully
understand garnet’s colorful mysteries!
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456
D ECEMBER 2013