Lets Talk Gemstones
By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this
article in any form.)
P.O.# 62653; COLORADO SPRINGS, CO. 80962
It has been noted earlier that the garnets are sometimes classified as the
Ugrandites, those species where calcium occupies the A site in the general
structural formula A3B2(SiO4)3], and the Pyralspites, those species where
the B site is occupied by aluminum. Little solid solution occurs between these
two categories, but solid solution series are formed between members of the
same category. Pyrope, almandine, and spessartine comprise the pyralspite
category. Solid solution series are formed between almandine and pyrope, pyrope
and spessartine, and between almandine and spessartine. A simple way to illustrate
the relationship of these species is to draw a triangle with lines connecting
the points representing the pure “end members” of the three species. Progression
along the line connecting the end members of two species indicates a ratio
change of the atoms in the elements that compose the chemical compositions
from the solid solution series’ end members. Such a change in the ratio often
affects the physical and optical properties of the varieties formed by the
change. Pure end members of the various series rarely occur. Therefore, the
intermediate varieties provide an important portion of the material for gemstones.
Such varieties often bear trade names that give no clue to their relationship
to the species. This “relationship triangle” provides the basis for the sequence
in which the pyralspite species and their varieties are presented.
This most common of the garnet species (an end member of two solid solution
series) occurs in a wide variety of locations and environments. Regional metamorphism
of clay sediments has made it an abundant constituent of metamorphic rocks
worldwide. Igneous rocks, alluvial deposits, and less frequent occurrences
in pegmatic granite are other sources. The major sources of gem material occur
in India, Sri Lanka, Australia and Brazil. Exceptionally large gem crystals
are produced from mines in Madagascar. Excellent material exhibiting asterism
is found in Idaho in the United States. [The Smithsonian Institution in Washington
D. C. has a 175-carat star gem from Idaho in its collection.] Slate deposits
near Wrangell, Alaska yield unusually well formed crystals.
Because pure almandine is rare, the term almandine is used when the iron
in its chemical composition exceeds that of magnesium in its solid solution
series with pyrope. The same is true of manganese in its solid solution series
with spessartine. Its iron content causes it to fuse at 3-3.5 to a magnetic
globule. Spectral examination always reveals a distinct and diagnostic pattern.
From 3 to 5 wide bands and a possible 2 or more lines create a vivid spectrum.
The presence of elements associated with pyrope and spessartine produce the
range of almandine’s hues from red, violet-red, orange-red, and reddish-brown
The asterism of the star garnets is caused by asbestiform pyroxene and amphibole
inclusions. Translucent and transparent almandine material frequently houses
short, stubby, low-relief crystals of biotite, ilmenite, spinel, quartz, apatite,
monazite, haloed metamict zircon crystals, rod-like hornblende, and asbestiform
needles of augite and acicular rutile crossed at 70 and 110 degrees. The
iron content and inclusions can cause variations of density from 3.95 to
4.30. Simon & Schuster’s Guide to Gems and Precious Stones states “the
density increases parallel to the (refractive) index”. Refractive indices
between 1.785-1.83 are the norm. Readings below 1.78 indicate an intermediate
member of the almandine-pyrope series. Joel Arem lists the dispersion as
0.027 in his Color Encyclopedia of Gemstones, and it is shown as 0.024 in
Walter Schumann’s Gemstones of the World. The vitreous luster sometimes borders
on adamantine. Despite its lesser hardness (6.5 – 7.5), almandine’s distinctive
fracture makes it much more effective than quartz for use in abrasive materials.
The major source for manufacture of these abrasives is the extensive garnet
deposit at Gore Mountain in New York. The condition of the huge crystals
there, extremely shattered by nature, precludes their use as gemstones.
Almandine’s name is a corruption of the name of the ancient city of Alabanda,
located in what is now southwest Turkey, near where it was discovered. It
is a gem with an ancient history. Noah is reputed to have hung garnet in the
ark to disperse light. It was used in windows of temples and cathedrals and
is believed to have been one of the gems representing the twelve tribes of
Israel decorating the “breastplate of Aaron”. The finest almandines from the
alluvial gemstone deposits in Sri Lanka are often called “Ceylon rubies”.
A careful examination of its physical and optical properties is sometimes
needed to distinguish almandine from red spinel. Paste (glass) is used as
a simulant. Almandine is not produced synthetically for use as gems. The expense
of production by the flux growth method is prohibitive, and the high temperature
of the melt method causes the conversion of the necessary ferric iron to
ferrous iron in the chemical composition with unsatisfactory results.
Intermediate Members of the Almandine - Pyrope Solid Solution Series
The chemical composition of the intermediate members of the series grades
almandine to pyrope as the presence of iron decreases and magnesium increases.
Without chemical analysis, the major criterion for determination and designation
of an intermediate member of the series is the refractive index. The refractive
index for the series exhibits a range from 1.785-1.830 for almandine to 1.714-1.742
for pyrope. The presence of chrome can usually be discovered by spectral analysis.
This gem garnet variety has increased enormously in popularity within the
last few years. Its purplish undertones have led to the use of the term “raspberry
garnet” in the trade. In The Illustrated Encyclopedia of Minerals and Rocks,
Dr. J. Kourimsky indicates the distinctive color “is caused by the presence
of iron and chromium”. Upon spectral examination, it exhibits the almandine
spectrum. Determination of its position in the series requires chemical analysis.
Without such analysis, most gemologists apply the name rhodolite if the refractive
index reading is 1.77 or below. In addition to the inclusions common to garnets,
apatite crystals are sometimes present.
The presence of chrome creates the especially vivid red of this intermediate
member of the series. Its position in the series lies closer to pyrope than
does that of rhodolite as the ratio of magnesium increases in the chemical
The red hues of the pyrope-almandine series nearest the end member pyrope
are commonly called pyrope. The Greek word pyropos, meaning “fiery-eyed”,
gives them the name. Volcanic rock and alluvial deposits in Argentina, Australia,
Brazil, Myanmar, Scotland, Switzerland, Tanzania, and the desert sands of
the southwest in the United States are sources of this popular garnet. Pyrope
often is associated with and appears as inclusions in diamonds in pipes located
in Africa and Canada. Pyropes are usually pea-sized or smaller. The slopes
of the Bohemian Highlands in Czechoslovakia are famous for the production
of especially fine pyropes called “Bohemian garnets”. Dr. Kourimsky tells
that Anselmus Boetius de Boot, in his Gemmarum et lapidum historia, “writes
not only about the garnet the size of a pigeon’s egg but about other large
pyropes” found there in the sixteenth century and that “pyropes the size of
a hazelnut were valued as highly as rubies”. Pyropes from this source were
used extensively in Victorian jewelry. “Cape ruby” is a name applied to the
pyrope found in deposits at the Cape of Good Hope in South Africa. Although
pyropes are notably free of inclusions, sometimes octahedra, tiny needles,
and rounded irregularly outlined quartz crystals (snowballs) are present.
Magnesium Aluminum Silicate
This end member of the two solid solution series almandine- pyrope and pyrope-spessartine
would be colorless in the pure state. Its refractive index range of 1.714
–1.742 is the lowest of all the garnets. The existence of pure pyrope as a
gem is unknown.
It is confusing that the term “pyrope” is used as a designation for an intermediate
member of the almandite-pyrope solid solution series. Neither iron nor manganese
is present in the chemical composition of this end member of the two solid
solution series, almandine-pyrope and pyrope-spessartite. However, it is
a component of the intermediate members of the two series and is present
with spessartite and grossular in the spessartite-grossular-pyrope type of
color-change garnet. The pure state of pyrope (it would be colorless) is
not known to exist in nature. A possible refractive index of 1.714 is the
lowest exhibited by a garnet. Joel Arem lists its physical and optical properties
as follows: possible variation of the refractive index of 1.730 to 1.766,
a dispersion of 0.022 and a variation of specific gravity of 3.65 to 3.87.
Intermediate members of the Pyrope - Spessartite solid solution series
As the chemical composition of the end member pyrope grades towards that
of the end member spessartite in the pyrope-spessartite series, the physical
and optical properties again change. The presence of manganese becomes more
pronounced. Intermediate members of this series can closely resemble the
hessonite variety of grossular garnet, but the granular “treacly” appearance
of hessonite under the microscope is absent. Brown, red, orange, and yellow
hues dominate the color range with peach and pink tones appearing as the
chemical make-up approaches the chemical composition of pure spessartite.
A dispersion of 0.027 combines with the high refractive index [1.79-1.81]
to make well cut gems of the intermediate members of this series especially
The name of this variety of the pyrope-spessartite series is not related
in any way to the area of southeast Asia known as Malaya or its people. It
is derived from the word of the Swahili tribe in Africa meaning prostitute.
Joel Arem in the Color Encyclopedia of Gemstones attributes it to the Bantu
tribe word for “outside the family” or “deceiver”. According to unconfirmed
reports, miners, frustrated in their search for a different mineral and disgusted
by the frequent presence of such a similar material, bestowed the unflattering
name on the similar substance. The sources in Africa provided the name, but,
perhaps, the use of the term pyrospessartite combined with the designation
of the place of origin, Tanzania, would create less confusion. As with most
varieties of garnet, it is necessary to closely examine pyrospessartite’s
physical and optical properties and/or subject the stone to chemical analysis
to make a positive identification. It can resemble hessonite of the grossular
garnets, but the diagnostic roiled “scotch in water” appearance of hessonite
under the microscope is absent. Colors range from brownish red to lighter
orange and yellow hues as the chemical composition nears that of pure spessartite.
Mn3Al2Si3O12 Manganese Aluminum Silicate
This end member of the two garnet series formed with pyrope and almandine
was discovered in the 1800s in the Spessart area in northwest Bavaria (Germany).
The deposit was small, and commercial exploitation of the material was limited.
Subsequently, sources of attractive materials (intermediate between pure
spessartite and almandite) were found in Sweden, Italy, Myanmar (Burma),
Sri Lanka, Pakistan, Australia, Mozambique, Kenya, Tanzania, Zambia, Madagascar,
Brazil, and the United States. The designation “spessartite” was applied
to all and continues today. A “pure” spessartite was unknown in the jewelry
trade until the discovery of the bright orange material in Namibia in 1991.
With a high refractive index of 1.79 to 1.81, spessartite ranks second only
to andradite in the wide range of indices exhibited by the garnet group.
The dispersion of 0.027 equals that of almandite and grossular but is less
than half that of andradite [0.057]. Specific gravity can vary from 4.12
to 4.18 with 4.15 being the norm. In addition to inclusions of tremolite,
irregular “cobwebs” caused by dispersed drops of included liquid may be present.
The discovery of the iron-free, brilliant orange spessartite in Namibia in
1991 in commercial quantities created a sensation in the jewelry trade. The
deposit lay in mica schist along the course of the Kunene river in the mountainous
northwest area bordering Angola. Few inclusions marred the excellent crystals.
The name “hollandine” was chosen for its introduction as a spectacular new
gemstone. This was changed to Mandarin garnet when it was discovered that
“hollandine” denotes a little-known metal. Within less than five years time,
the deposit was depleted, and the area closed. The planned recovery of material
from metamorphic bedrock in the rugged terrain surrounding the first find
will be more difficult. Crystals recovered from the surface layer of this
area contain numerous inclusions of tremolite, but cleaner material lies
below. In 1999, a new source of the pinkish-orange gemstone was discovered
in a remote area of Nigeria. Though the material was more yellow than the
orange Namibian crystals, the size and abundance of the material made the
public more aware of and increased the demand for spessartite gems. Despite
great expectations, this deposit, too, was quickly depleted. Acquisition
of material from the Nigerian area is sporadic and available only in small
amounts from local native traders. Currently, developers are investigating
a promising new deposit of a high (reputedly 90% or more) manganese content
material located in the gem-rich Alto Mirador pegmatite dike of Paraiba in
Brazil. Mandarin garnet brought spessartite wide acclaim. It is confusing
that the terms “spessartite” “spessartine” and “mandarin garnet” are frequently
applied to intermediate members of the spessartite-almandite garnet series.
Iron-bearing Intermediate Members of Spessartite-Almandite Series
As the ratio of iron to manganese increases and the chemical composition
grades from spessartite to almandite, the refractive index rises and brown
tones appear. However, the substances retain a vivid color caused by the
presence of the manganese. This is typical of gems from Madagascar and the
other sources noted above. The orangy-red material produced in Amelia, Virginia
and Ramona, California set the standard for the most desired color for such
spessartite gems until the Namibian discovery. Opened in 1903, the Little
Three and the Hercules mines situated in a pegmatite dike near San Diego,
California were the best known sources for iron-bearing spessartite gems
until the late twentieth century. The high cost of production caused the
close of the mines in 1997. As noted above, the intermediate members of the
spessartite-almandite solid solution series are frequently and incorrectly
referred to as spessartite or spessartine or mandarin garnet. These terms
are no longer exclusive to iron free spessartite.
It has been noted earlier that the chemical composition of garnet can vary
widely, so long as the “size” of an atom can be accommodated within the structural
lattice. An intermediate member of a garnet series is often said to contain
a “mixture of the molecules” of the end members of the series. This is a
simple way to say that the ratio of chemical elements in the chemical composition
changes in a progression from one end member to the other. In the pyrope
to spessartite series, the elements involved are magnesium and manganese.
In the chemical composition of pure pyrope, there is no manganese. If manganese
is present in the surrounding matter as a garnet crystal develops, then it
may occupy an atomic site in the lattice that could be occupied by a magnesium
atom. It is thus said that a manganese atom has replaced an atom of magnesium.
As the ratio of manganese to magnesium increases, the series “grades” from
the end member pyrope to the end member spessartite until manganese has completely
“replaced” magnesium in the chemical composition. It is important to remember
that atoms of other elements can occupy sites in the atomic lattice. The
color-change garnets are excellent examples of the complexity of the chemical
make-up of the garnets.
The color-change garnets found in East Africa involve “a mixture of the molecules”
of spessartite and grossular, with a substantial amount of chrome and vanadium
incorporated into the chemical composition. Spessartite is the major component,
but grossular can be almost half the composition, with almandine or pyrope
also in the mix. In the Color Encyclopedia of Gemstones, Joel Arem gives
the following information. The spessartite-grossular-almandine color-change
garnet has a refractive index of 1.773, with a density norm of 3.98. It appears
greenish-yellow brown in transmitted fluorescent light. In reflected fluorescent
light, it changes to purplish-red. In incandescent light, it exhibits a reddish-orange
to red color.
The spessartite-grossular-pyrope color-change garnet exhibits a change of
color from “light bluish green” in transmitted fluorescent light to purple
in reflected fluorescent light. In incandescent light, the color is “light
red to purplish red”. The refractive index is 1.763 with a specific gravity
norm of 3.89.
Again, the Color Encyclopedia of Gemstones is the source of the information
concerning the pyrope-spessartite color-change garnet found in the Umba Valley
of East Africa. Calcium and titanium are a part of its chemical composition.
A refractive index of 1.757 and a specific gravity of 3.816 are normal. The
spectrum exhibits “absorption bands at 4100, 4210, and 4300 that may merge
to form a cutoff at 4350.” A wide definite band at 5730 occurs in material
that exhibits a strong change of color. Acicular rutile and hematite platelets
are common inclusions. These gems change from reddish purple in tungsten
light to greenish-blue in daylight.
Joel Arem mentions color-change garnet crystals of less than a carat that
have been recovered in Norway. The material exhibits a refractive index of
1.747 with a density of 3.715. The color changes from “violet in daylight”
to “wine-red in incandescent light.” He also mentions “alexandrite-like garnets”
with a color change from “violet-red to blue-green.” He notes only that although
they “are small, -- a stone of 24.87 carats was sold in 1979”.