MINISTRY OF MINERALS

MINERAL RESOURCES INSTITUTE

DEPARTMENT OF GEOLOGY AND MINERAL EXPLORATION

PRESENTATION SESSION: GEMOLOGY

PRESENTER NAME; YEDIDIA MGEMA

TITLE; GEOLOGIST,GEMS TRADER AND MINERAL ECONOMIST

SPINEL GEMSTONE
LIST OF TABLES
Figure 1: Represents Colored Spinel Optical Properties ................................................................ 6
Figure 2: Represents shape appearance of Spinel Optical Properties ............................................. 6
Figure 3: Represents Winza Spinel.............................................................................................. 21
Figure 4: Represents Mahenge Spinel .......................................................................................... 22
Figure 5: Represents the location of Spinel’s discovery .............................................................. 23
Figure 6: Represents Spinel decoration from Ruby ...................................................................... 24
Figure 7: Represents the Blue Gahnite from Nigeria.......................................................................... 25
Figure 8: Represents rough octahedral ......................................................................................... 27
Figure 9: Different forms and types of Spinel at Buma and Mahenge (Tanzania)....................... 28
Figure 10: RepresentsMahenge Spinel and Diamond Ring in 18k White Gold ........................... 29
Figure 11: Represents Spinel Market in Chanthaburi Thailand ................................................... 30
Figure 12: Represents Workers sorting SPINEL concentrate from the Baw Mar mine near
Mogok. .......................................................................................................................................... 31
Figure 13: Represents Talad Ploy exported gemstones worth $650 million in 2012 ................... 32
Figure 14: Represents Queen Elizabeth II in 2016 wearing the Imperial State Crown. A large
uncut red spinel, called Black Prince’s Ruby, is set into the front. .............................................. 33
 LIST OF TABLES
Table 1: Represents the Spinel Physical Properties ........................................................................ 5
Table 2: Represents the type and Isotropic of Spinel Optical Properties ....................................... 7
Table 3: Represents the Chemical Properties of Spinel Optical Properties .................................... 7
SPINEL
Spinel is a gemstone that comes in a variety of colors. Pure Spinel is colorless, but impurities are
responsible for the wide range of colors. The most valuable and desirable Spinel color is a deep
blood red, though blue, yellow, orange, pink, and purple colors also make fine gemstones.

Table 1: Represents the Spinel Physical Properties

Spinel Physical Properties

Various; red, pink, blue, lavender/violet, dark green, brown, black,

Color
colorless

Streak White

Luster Vitreous

Cleavage None

Diaphaneity Transparent to Opaque

Moh’s Hardness scale 7.5–8.0

Specific Gravity Zn-rich can be as high as 4.40, otherwise it averages from 3.58-3.61

Diagnostic Properties Hardness, octahedral crystals, vitreous luster

Crystal System Cubic

Tenacity Brittle

Separation plane {111} indistinct and probably represents parting

Parting
rather than cleavage.
 Spinel Optical Properties

Figure 1: Represents Colored Spinel Optical Properties

Figure 2: Represents shape appearance of Spinel Optical Properties

Table 2: Represents the type and Isotropic of Spinel Optical Properties

Type Isotropic

Color / Pleochroism Non-pleochroic

Twinning common

Birefringence Isotropic minerals have no birefringence

Table 3: Represents the Chemical Properties of Spinel Optical Properties

Chemical Classification Oxide minerals

Chemical Composition MgAl2O4
Mineral Group: Spinel group
Polymorphism & Series: Forms three series, with magnesiochromite, with
gahnite, and with hercynite.

Crystallography. Isometric; hex octahedral. Usually in octahedral crystals. In

“twinned octahedrons (spinel twins). Dodecahedron may be present as small
truncations. Other forms rare.

Composition: MgAI20 4. MgO = 28.2 percent, A120 3 = 71.8 percent. The

magnesium may be replaced, in part, by ferrous iron or manganese, and the
aluminum by ferric iron and chromium. The clear red, nearly pure, magnesium
spinel is known as ruby spinel. Pleonaste is the iron spinel, dark green to black;
and picotite is the chrome spinel, yellowish to greenish brown

Diagnostic Features: Recognized by its hardness (8), its octahedral crystals,

and its vitreous luster. The iron spinel can be distinguished from magnetite by
its nonmagnetic character and white streak.

Varieties And Trade Names of Spinel

Because spinel is so little known, it is the contention that the species should be
described by color terms; instead of attempting to popularize variety names.
However, in the following list, the old variety names are mentioned for
purposes of assisting the student in recognizing them when they are
encountered in older literature.

Red Spinel, Ruby-Colored Spinel, or Ruby Spinel

A misleading term, "spiel ruby" has been used for this variety. The best colors
are those that approach the appearance of fine ruby, although very dark garnet
reds to light purplish reds bordering on pink are included under this heading.

Purple, or Almardine, Spinel

Purple to red-purple.

Pink, or Rose, Spinel

Tones of light red to light purplish red. A misleading term for these colors that
was formerly used is "Balas ruby", although in older literature it was used for
all red to light-red spinel.

Orange Spinel

This color was formerly called rubicelle (from the French "rubace", a form of
"rubis", meaning "ruby"). Another name for the brightest of these yellow-
orange to orange- red colors is flame spinel.

Blue Spinel, Sapphire-Colored Spinel, or Sapphire Spinel.

Rarely is this vaRarely is this variety a fine blue; more often it is a grayed-out
dark blue to violet-blue or greenish blue. Even the best of these are collectors’
items, rather than commercial stones. Except by taking an R.I. or S.G.,
gahnospinel is indistinguishable from ordinary blue spinel. An older term,
"sapphirine", is misleading and should be discontinued.
Alexandrite like Spinel

This is a comparatively rare spinel in which the color is a light grayish blue in
daylight and an amethystine violet under artificial light.

Black Spinel

This opaque spine is rarely This opaque spine is rarely seen now that mourning
jewelry is out of fashion, although it has appeared in some modern calibre
work, such as guard rings.

Other colors of spinel would be called by the color involved. Thus, colorless
and yellow spinels have been reported but not authenticated by the Institute.
Green spinel, except in unattractive and nearly opaque stones, is rare and has
been called CHIDRSPINEL, a term of little use to the practical gemologist.

Star spinels have been reported and described in gemological journals.

However, a search of the literature seems to limit the number so reported to
fewer than ten stones. The three stones seen by the Institute's staff were
imported from Ceylon as "black star spinel sapphires". The stones were
determined to be spinel, with rutile needles as the cause of the star. The rutile
needles are oriented parallel to the edge of the regular octahedron. The
inclusion causing the star in other spinels have been identified as acicular
sphene crystals. Most of the stones have been described as having a six-rayed
star on the apex of the cabochon, with alternating six and four-rayed stars at
the girdles (see illustration). All but one of these stars have been described as
dark purple to black in color. The exception was described as a gray stone from
Burma.

Formation of Spinel

Almost all gem spinels have been formed by contact metamorphic activity
associated with intrusions of molten rock Masses into impure dolomites.
However, as with so many gemstones that come from Burma and Ceylon, the
primary source is not mined, for it requires the concentration effected by nature
in alluvium to make deposits rich enough to mine. Spinels of non gem quality
occur in certain aluminum-rich basic igneous rocks, as well as in deposits that
arise from the metamorphic alteration of these rocks.

The spinels from the countries mentioned above are associated with corundum
and undoubtedly were formed in the same rocks as the rubies and sapphires.
Since both spinel and corundum are durable minerals, they occur together in
the gem gravels.

Occurrence

It is a common metamorphic mineral occurring imbedded in crystalline

limestone, gneisses, and serpentine. Occurs also as an accessory mineral in
many basic igneous rocks. It is frequently formed as a contact metamorphic
mineral associated with phlogopite, pyrrhotite, chondrodite, and graphite.
Found frequently as rolled pebbles in stream sands, where it has been
preserved because of its resistant physical and chemical properties. The ruby
spinels are found in this way often associated with the gem corundum, in the
sands of Ceylon, Siam, Upper Burma, and Madagascar. Ordinary spinel is
found in various localities in New York and New Jersey

Formation of Spinel

Almost all gem spinels have been formed by contact metamorphic activity
associated with intrusions of molten rock Masses into impure limestones or
dolomites. However, as with so many gemstones that come from Burma and
Ceylon, the primary source is not mined, for it requires the concentration
effected by nature in alluvium to make deposits rich enough to mine. Spinels
of non gem quality occur in certain aluminum-rich basic igneous rocks, as well
as in deposits that arise from the metamorphic alteration of these rocks.

The spinels from the countries mentioned above are associated with corundum
and undoubtedly were formed in the same rocks as the rubies and sapphires.
Since both spinel and corundum are durable minerals, they occur together in
the gem gravels.

Sources of Spinel

In medieval times the source of fine spinel was the ruby mines in the
Badakshan region of Afghanistan, where it was mined in alluvial deposit.
Possibly this source accounts for the very large stones of hi-story and in
collections and museums. Today, it cannot be considered a source of either
ruby or spinel.

Burma

At present, Burma is undoubtedly the source of the best gem-quality spinel,

just as it is known for the quality of its rubies. Here the spinel is mixed with
the rubies in the gem gravels, although not in the same proportions in the
various mines. Oddly enough, the rubies are usually badly waterworn, so that
few crystal faces are exhibited, whereas many of the spinel crystals occur as
excellent octahedral or twins that show only slight abrasion. No completely
satisfactory explanation has been offered for this phenomenon, since corundum
is appreciably harder (9) than spinel (8). In the Burma deposits, spinels of
brilliant red, orange and pink, as well as the more somber blues and violets, are
found. Fine blue stones have been reported, but they seldom leave the hands of
local collectors, in whose collections travelers have reported seeing them.

From 1597 until 1887, when the Burma Ruby Mines, ltd., a British company,
began production, the Burma deposits were mined in a primitive manner by
licensed miners. More mechanization, drainage canals and other modern
improvements were used by the British company. However, this firm suffered
much adversity after the first few lucrative years, and the final closing in 1931
marked the end of its career. Since that time the area has been mined by
natives in a manner that differs little from the primitive methods used 400
years ago.

Since the spinels recovered are, in effect, a by-product of the search for the
more valuable corundum gems, ruby and sapphire, the mining methods
outlined in that assignment apply equally to spinel.

Ceylon (Sri Lanka)

Spinel that occurs in the gem gravels in Ceylon tends towards the blue and
violet hues. However, good red and pink spinels are recovered occasionally
that vie with any from Burma for quality. As the name suggests, black
Ceylonite spinels are recovered here, as well as some very dark-green to
brownish-green stones, which are rarely encountered in the market. In Ceylon,
as in Burma, the spinels are recovered more or less as a by-product of the
mining of gem gravels for the more valuable sapphires, star sapphires and the
rare Ceylon rubies. The mining process is described in detail in both the
corundum and the zircon assignments.

A few spinels are recovered in Thailand and the Pailin area, which borders on
Cambodia. Here, as in other areas, the primary target of the miners is not spinel
but the more valuable, transparent sapphires and the popular black-star
sapphires. The deposits are alluvial, as they are in Burma and Ceylon. Spinels
are also found in Thailand near

(Chant bur) an area renowned for its dark rubies and fine sapphires in alluvial
gravels.

Other Sources
A few dark-blue spinel crystals have been recovered at the base-metal mines at
Jemaa, Nigeria, Africa. Other localities are of mineralogical interest only, and
have not provided the trade with any marketable stones.

Spinel Jewelry

Most spinels that reach the jewelry trade have been polished in their country of
origin. Because color is the main consideration, the cuts employed are not
usually precise; therefore, the fine colors that warrant the expense are usually
re-cut in the United States. A characteristic of many commercially-cut spinels
is their tendency to be too shallow, because many of the best crystals are
flattened octahedral or twins. These twins are so characteristic of spinel that
when they occur in other cubic minerals, such as diamond, they are sometimes
called spinel twins by crystallographers. When the proportions are good, the
brilliancy of the lighter colored stones is excellent. This brilliance is partly
explained by the nature of the polished surface. Because of the heat developed
during the polishing operation, most stones develop a liquid like layer on the
polished surface, which, upon solidification, usually crystallizers. Spinel,
however, is an example of a gemstone on which this layer remains amorphous
and therefore smoother than a finely crystalline surface. This layer is called the
"Beilby layer", after Sir George Thomas Beilby, a British physicist.

Although most spinels worth cutting are faceted, the rare star spinel is always
cut in cabochon. Flawed but attractively colored material is occasionally
fashioned in the cabochon style or used for polished beads for necklaces or
rosaries. Since spinel is not heat sensitive, only the usual care is required in the
various operations.

No special orientation is required, unless it is necessary to eliminate or

minimize flaws. Performing is accomplished on coarse and fine silicon-carbide
grinding wheels. Grinding the facets may be done on a copper lap with
diamond powder or on a lead lap with fine silicon-carbide powder. It polishes
well on a tint lap with either tin oxide or Linde A powder as the polishing
agent. A type-metal lap with Linde A also gives good results. Although
polishing the facets seldom proves troublesome, a change of polishing
direction will usually overcome any difficult that may be encountered.
Suggested angles are 37° for the crown facets and 42° for the pavilion facets.
Spinel lends itself well to the step or emerald cut; however, many are cut in the
brilliant style.

Physical & Optical Properties of Spinel

A. Physical Properties

Chemical Composition A double oxide of magnesium and aluminum,

expressed by the formula MgAl2O4.
Crystallographic Cubic system. Habit: Octahedral and twinned
Character tones. Most gems are found as water worn
pebbles.

Hardness 8
Toughness Good
Cleavage Poorly developed. Cleavages parallel to
octahedral faces may rarely occur, but they are
too difficult to detract from toughness.
Fracture Conchoidal
Specific Gravity 3.57 to 3.90; normal 3.60; Gahnospinel 4.01;
Ceylonite, 4.01; and gahnite 4.0 to 4.6.
Streak White. Very dark stones may show a tinted
streak.
Characteristic Minute octahedral crystal arranged either singly
Inclusions or in "fingerprint" patterns. Stones may be
flawless, of course.
 B. Optical Properties

Degree of Transparency Gem-quality material is transparent to

semitransparent. Ceylonese and gahnite may
be opaque. Star spinel is semi-translucent
Luster Polished surfaces are sub adamantine; fracture
surfaces are vitreous.
Refractive Index Gem material is usually 1.718 and almost
always between 1.715 and 1.720, but very
rarely as high as 1.735. Gahno spinel, 1.74 to
1.78; gahnite, 1.80.
Birefringence None
Optical Character Isotropic (singly refractive).
Pleo chroism None
Dispersion 0.20
Phenomena Asterism encountered rarely; change of color;
fluorescence
X-Ray, Fluorescence Red and pink stones glow red; other colors are
inert.
Ultraviolet Fluorescence Red and pink stones glow red under long
wavelength radiation in direct relation to depth
of color, with the exception of dark-red stones,
which glow weakly Red stones show sharp
lines in the red at 6855 and 6840 A.U., a weak
band at 6560, and a strong absorption in the
yellow-green from about 5950 to 4900 A.U. In
the pink and bright-red stories, the spectrum
may show a series of five bright-red
fluorescent lines in the red. The spectrum is
caused by chromium
 Absorption Spectra Blue stones show a band of medium strength
in the orange at 6320 A.U.; a somewhat
stronger and narrower band at 5550; and a
strong broad band at 4590, with a narrow line
at 4800. More or less complete absorption
occurs below 4300. Many stones show the
bands in the yellow and orange so weakly as
to be nonexistent. The spectrum is caused by
ferrous iron. Other colors, such as violet and
purple, may show the ferrous spectrum
weakly, depending on the strength of the blue
component of the color.

Gahno spinel and gahnite tend to have the

same spectrum as blue spinel, only much
stronger. The 5920 line in the yellow and the
5550 line in the green may be quite distinct,
whereas the 4590 band may be so broad as to
over shadow the 4800 line.

Effects Caused by:

Heat Light colored stones may fade under intense heat. Spinel
melts at approximately 2135° C.

Acids Insoluble.

Irradiation No data available

Test and Identification of Spinel

By sight alone, it is possible to confuse spinel with other stones of similar

color. However, aside from a rare pyrope garnet with a low R.I. and synthetic
spinel, normal use of the refractometer and polariscope will identify spinel.
With the rare pyrope, the use of either the spectroscope or ultraviolet light will
separate them. All but the darkest red spinel will fluoresce under ultraviolet,
whereas garnet never does. The absorption spectrum, particularly because of
this fluorescence is different in the two minerals. The differences were
explained in detail in Assignment of Spectroscope.

In general, any synthetic spinel with the color of a natural spinel will show an
R.I. close to 1.73, rather than 1.72. In the polariscope, the synthetic, because of
a slightly abnormal chemical composition and a consequent strained crystal
structure, will show a patchy extinction. It may be necessary to hold a 10x
loupe over the stone in order to see this clearly, especially in darker stones.
Natural stones may show anomalous double refraction in the form of a very
slight lightening and darkening when rotated in the polariscope, but not the
patchiness of the synthetic. Natural spinel often seems entirety or very nearly
devoid of the anomalous double refraction shown by most of the other singly
refractive gem materials. The presence of octahedral inclusions proves natural
origin; gas bubbles, either round or in long "gas hoses", proves synthetic
origin. Gas bubbles in the synthetic are usually exceedingly small, and often
appear to have a faintly octagonal outline. Usually, the colors of the synthetic
are unlike those of the natural stones, being used, as mentioned previously, to
imitate other gemstones, rather than natural spinels. Completely colorless,
yellow, and marketable green spinels are almost certainly synthetic. A deep-
red synthetic spinel is produced, but it is rarely seen on the market; its R.I. can
be expected to be close to 1.721.

Richly colored blue substitutes for the Kashmir quality of sapphire are almost
certain to be synthetic spinel, rather than synthetic sapphire. They are
characterized by red overtones resembling strong red fire. These stones appear
red through an emerald filter.

The possibility of glass having an R.I. similar to that of spinel exists. However,
in order to bring the R.I. of glass to 1.72, a great deal of lead has to be added,
which makes the S.G. much higher than that or spinel. Furthermore, it makes
the stone too soft for jewelry use. Therefore, glass with an R.I. above 1.70 is a
curiosity as a cut stone and is rarely encountered. Garnet and glass doublets are
easily distinguished by magnification; they never have a garnet cap of an index
in the gem spinel range.

A gahnospinel with an R.I. near that of sapphire does occasionally appear. The
single refraction and lack of dichroism will separate it from sapphire, if the
peculiar grayed-out color does not give it away by eye.

Valuation of Spinel

The most highly prized colors in spinel are undoubtedly the intense red to
purplish red and the orange-red. Actually, many hues exist in the red to
purplish-red category, varying from those that can be mistaken for pyrope or
almandite to those that might be mistaken for ruby and those that are more
nearly the color of pink sapphire. The connoisseur seldom confuses ruby and
red spinel. The great clarity of this color of spinel, together with its lack of
"dichroism and the evenness of its color differentiates it from ruby, which in
addition to its dichroism, often has a splotchy or banded color. Usually, the
beauty and rarity of fine red spinel is not reflected in the price asked for it. This
may be said also for the intense, orange-red flame spinel, which is the more
costly of the two.

The other colors of spinel can only be judged by their individual desirability,
since, except for pink or very light violet; the colors tend to be too dark and
grayed out. In this regard, it is well to note that the purple and dark-blue stones
may be very transparent in transmitted light but grayed out when viewed in a
setting. Because of the availability of brighter sapphires and because of the
small sizes of spinel in these grayed-out colors, they have never been popular.
In all fairness, it should also be noted that an attractive violet to light-purple or
bluish green stone will be encountered occasionally. The rare alexandrite like
spinel, which changes from a light grayish blue in day ling to an amethystine
violet in artificial light, should be judged by the quality of each color and the
degree of change under the two light sources. None of these colors commands
a high price, unless it has been mistaken for corundum. When the identity is
known, the price usually drops.

In general, faceted spinels are much more flawless than the corundum gems,
especially when judged with the unaided eye. Visible flaws, therefore, affect
value considerably, particularly if they are in the form of cracks or structural
defects. Certain stones with especially desirable color may not be affected in
value greatly by the presence of inclusions that are visible to the unaided eye;
however, this usually does not hold true with less desirable colors.

Because of the rarity of large, rough spinel and the fact that many crystals are
twins or flattened octahedral, the finished stones are often cut with other than
ideal proportions. The color of a stone may compensate partly for the cutting
deficiencies. However, the quality of cutting in stones with other than fine
color must be judged by observing the degree of brilliancy and the area of light
leakage ("dead" area, or "window") near the culet. The smaller this area, the
more desirable the stone from a cutting standpoint. Spinel will take an
exceptionally fine polish with ease, lapidaries report; hence, stones with
obvious polishing deficiencies are not as valuable as well-polished stones. One
factor that does not require consideration when judging spinel is orientation.
Since the color is virtually always evenly distributed throughout and there is no
dichroism, the lapidary can cut the stone with a view to eliminating flaws and
obtaining as large a stone as possible without the complicating factors of
orientation and color banding.

Large spinel that is, stones of good color in sizes over ten carats are rare
indeed. Museums are happy to have a fine red specimen to display, even if it
weighs less than ten carats. In this regard, it is more fortunate than ruby, since
very few ten carat rubies are on display. In spite of the rarity of large sizes, the
price per carat does not increase as rapidly for the larger sizes as it does for
ruby or diamond, although large red spinel may be very desirable and
beautiful. Size makes very little difference in price per carat in the colors other
than red, orange and pink.

WINZA SPINEL DEPOSIT IN TANZANIA

GEOLOGIC SETTING The Winza area belongs to the Paleoproterozoic

Usagaran Belt. A rock unit composed of highly metamorphosed basement
rocks, metasediments that have undergone a lower grade of metamorphism,
and felsic magmatic intrusives and volcaniclastic sediments that are nearly
unmetamorphosed and undeformed. Structurally, the Usagaran Belt constitutes
the eastern border of the Archean Tanzania Craton (see, e.g., Fritz et al., 2005).
The eastern parts of the Usagaran Belt were reworked during the
Neoproterozoic East African Orogeny and have been designated the Western
Granulites, while further to the east, a unit of younger Mesoproterozoic rocks
composed of enderbitic gneisses (metamorphosed igneous rocks of the
charnockite series), schists, and marbles has been named the Eastern
Granulites (Fritz et al., 2005). The Eastern and Western Granulites were
metamorphosed simultaneously, and together they constitute the
Neoproterozoic Mozambique Belt of Tanzania and Kenya. Most of the known
gem corundum occurrences in Tanzania and southeastern Kenya—including
Longido, Umba, the Mangari area, Morogoro, and Mahenge—belong to the
granulite-facies Eastern Granulites, which was overprinted by a metamorphic
event some 620–640 million years ago (Ma) at temperatures around 750–
850°C and pressures in the range of 9.5–12 kbar (Appel et al., 1998; Moeller et
al., 2000; Hauzenberger et al., 2004, 2007).
Figure 3: Represents Winza Spinel

The Stone That Changed Everything

 Figure 4: Represents Mahenge Spinel

Mahenge Spinel is the stone that changed every ones opinions about Spinel. It is the sole reason
why so many people around the world are now familiar with the beauty of the Mahenge Spinel.
The vibrant pinks and the deep reds combined with durability that is only surpassed by a handful
of gemstone have made Mahenge Spinel one of the most popular stones of the new age.

Spinel in History

For centuries, Spinel has been confused with the other great red gemstone, the Ruby. Spinel is
often found in the same deposits as Ruby, so for a long time it was impossible to identify them as
a separate gemstone. Now with modern testing techniques, these two stones are easily
identifiable. Before the discovery of the Mahenge Spinel the most beautiful red Spinels had
originally come from the Balas region in Afghanistan. These were called Balas Rubies since
their identification was confused.
 Figure 5: Represents the location of Spinel’s discovery

One of the most famous cases of mistaken identity is the “Black Princes” Ruby that is part of the
Royal Crown Jewels. This Ruby is actually a big beautiful red Spinel.
 Figure 6: Represents Spinel decoration from Ruby

Discovery of Mahenge Spinel

In 2007 the world of Spinel was as about to change. In a farmers field in Mahenge, Tanzania,
workers found a giant Spinel crystal weighing 52 Kgs (114 Pounds). This Mahenge Spinel
crystal displayed the gemmy vibrant pink color that is now famous from Mahenge Spinel. The
crystal was highly included, but large chunks of top quality stone were able to be cobbed of the
side. This resulted in thousands of carats of gem quality Mahenge Spinel being cut in Thailand
and distributed to the world market. From this point on, the Mahenge Spinel was thrust into
spotlight as one of the most beautiful and vibrant gemstone available on the market today.

Spinel Durability - The Underdog

Today the properties of Spinel are still not widely know by the public. The thing that sets Spinel
up to be one of the best gemstones for everyday wearing is its durability. In terms of hardness it
sits at an 8 on the MOHS scale. This means that the only stones that are harder than Spinel are
Sapphires, Rubies and Diamonds. This makes it extremely resistant to scratching so the
gemstone will hold its shine for a long time. It is also very hard so it can withstand shock. Both
of these properties make it a perfect choice for Jewelry that is to be worn every day. It is even
durable enough to be considered for an engagement ring!

The Mahenge Spinel is a special stone and it redefined the definition of Spinel. It is famous for
its vibrant intense pink color that is free from any secondary hue such as the grays commonly
seen in Sri Lankan Spinel. The color of these gems is also nice and consistent under different
lighting conditions. This makes it a perfect gem to be worn every day.

Figure 7: Represents the Blue Gahnite from Nigeria

Three single octahedral of natural gahnite, approximately 1.5 carat each, from Kaduna State,
Nigeria.In recent years, Nigeria has gained considerable attention in the gem trade for the
discovery of sapphires reportedly from the Mambilla Plateau in Taraba State. In addition to
sapphires, spinels have been sporadically mined in Jemaa and recently in Kagoro, both in
Kaduna State. The spinel group of minerals, with a general chemical formula AB2O4, has a total
of 22 different species. Four of these species have Al predominantly occupying the octahedral
site: spinel (MgAl2O4), galaxite (MnAl2O4), hercynite (FeAl2O4), and gahnite (ZnAl2O4). The
spinels from Kagoro have been identified as gahnite, the zinc end member. Gahnite is one of the
rarer members of the spinel group of minerals, typically found at zinc deposits. Most crystals are
very small, included, and translucent to opaque. Faceted gems are very collectible due to their
rarity and color.
The geology of the Nigerian spinels has been previously studied (R. Jacobson and J.S. Webb,
“The pegmatites of central Nigeria,” Geological Survey of Nigeria Bulletin, No. 17, 1946, pp. 1–
61), and the deposits can be subdivided into three groups based on their mineralogy:

(1) microcline-quartz pegmatites, which commonly occur within the calc-alkaline granitoids and
are rarely mineralized;

(2) microcline-quartz-mica pegmatites, found within metasedimentary sequences; and

(3) quartz-mica veins, which occur in schists and gneisses or marginal to group 2 pegmatites.
Gahnite occurs in groups 2 and 3.

Knowledge of the composition of zinc-rich spinels is useful in separating a wide variety of

source environments. For example, Zn content is high in spinels occurring in rocks that have
experienced low-grade metamorphism and high oxygen and sulfur fugacity (fO2 and fS2),
whereas Zn content is lowest in high-grade metamorphic rocks (A. Heimann et al., “Zincian
spinel associated with metamorphosed Proterozoic base-metal sulfide occurrences, Colorado: A
re-evaluation of gahnite composition as a guide in exploration,” Canadian Mineralogist, Vol. 43,
No. 2, 2005, pp. 601–622).

Standard gemological examination of three rough octahedra gave the following properties:
color—blue; pleochroism—none; refractive index—1.791 to over the limit (flat reading from
polished crystal face); hydrostatic specific gravity—4.180–4.294; fluorescence reaction—inert to
both long- and short-wave UV; and color filter reaction—red. Internal characteristics observed
with a gemological microscope were colorless unidentified crystals, fingerprints, and a red-
brown crystal inclusion (Raman identification of this inclusion was consistent with sphalerite)
 Figure 8: Represents rough octahedral

.
SPINEL DEPOSITS IN MADAGASCAR
Madagascar is one of the most important gem-producing countries in the world, including ruby
and sapphires. Gem corundum deposits formed at different stages in the geological evolution of
the island and in contrasting environments. Four main settings are identified: (1) Gem corundum
formed in the Precambrian basement within the Neoproterozoic terranes of southern Madagascar,
and in the volcano-sedimentary series of Beforona, north of Antananarivo. In the south, high-
temperature (700 to 800 °C) and low-pressure (4 to 5 kbar) granulites contain deposits formed
during the Pan-African orogenesis between 565 and 490 Ma. They accompany mafic and
ultramafic complexes (ruby deposits of the Vohibory group), skarns at the contact between
Anosyan granites and the Proterozoic Tranomaro group (sapphire deposits of the Tranomaro–
Andranondambo district), and shear-zone corridors cross-cutting feldspathic gneisses,
cordieritites and clinopyroxenites in the Tranomaro, Vohimena and Androyan metamorphic
series (biotite schist deposits of Sahambano and Zazafotsy, cordieritites of Iankaroka and
Ambatomena). The circulation of fluids, especially along discontinuities, allowed in-situ alkaline
metasomatism, forming corundum host rocks related to desilicified granites, biotitites,
“sakenites” and “corundumites”. (2) Gem corundum also occurs in the Triassic detrital
formations of the Isalo group, as giant palaeoplacers in the Ilakaka–Sakaraha area. Here,
sapphires and rubies may come from the metamorphic granulitic terranes of southern
Madagascar. (3) Gem corundum deposits occur within the Neogene-Quaternary alkali basalts
from Ankaratra (Antsirabe–Antanifotsy area) and in the Ambohitra Province (Nosy Be, Ambato
and Ambondromifehy districts). Primary deposits are rare, except at Soamiakatra where ruby in
gabbroic and clinopyroxenite xenoliths within alkali-basalts probably derive from mantle garnet
peridotites. The blue-green-yellow sapphires typical of basaltic fields are always recovered in
palaeoplacer (in karst formed upon Jurassic limestone’s from the Montagne d'Ambre,
Antsiranana Province) and alluvial and soil placers (Ankaratra volcanic massif). (4) Deposits
occur within Quaternary eluvial, colluvial and alluvial concentrations, such as high-quality
rubies from the Andilamena and Vatomandry deposits.

Spinel Gemstone Prices

Spinel has long been a favorite of gem dealers, but in recent years it has become very popular
with gem collectors and jewelry lovers as well. Because the supply of natural spinel is very
limited, you are unlikely to find it at retail jewelry shops. The spinel trade is mainly the province
of specialized gem dealers who trade in the finest stones.

Red Spinel Pink Spinel Red Spinel Pink Spinel

Burma Burma Mahenge, Tanzania Mahenge, Tanzania

Figure 9: Different forms and types of Spinel at Buma and Mahenge (Tanzania)

Prices on fine spinel have risen with increased demand, though spinel prices continue to be
attractive relative to ruby and sapphire. When you consider that almost all the spinel is the
market is completely untreated, it is still quite a bargain compared to prices for unheated
corundum. Because spinel has excellent hardness (8 on the Mohs scale), it is an gem suitable for
all types of jewelry.
Spinel prices are based on color, clarity, size and, to some degree, on origin. Spinel occurs in a
wide range of colors, including red, pink, orange, yellow, green, blue, gray and black. Star
spinels are known, but are extremely rare. Certain colors are more valuable, especially reds and
strong pinks. Many spinels have a significant gray secondary hue, and these colors are much less
valuable.

Figure 10: Represents Mahenge Spinel and Diamond Ring in 18k White Gold

The main sources for spinel are Burma, Sri Lanka, Tanzania and the Badakshan region on the
border of Afghanistan and Tajikistan. Until recently, the highest prices were paid for red spinel
from the Mogok region of Burma, particularly for pure reds over 2 carats. But in 2007 some
superb pink-red spinels were discovered in Mahenge, Tanzania. These extraordinary gems have a
neon-like glow that is unique in the gems world, and these Mahenge spinels are now regarded as
the best in the world.

Fine spinel’s from both Burma and Tanzania are still very affordable in sizes under 1 carat, with
prices ranging from $200 to $500 a carat for good colors. But prices go up exponentially with
size. Top reds and pinks typically sell for $600 to $1,200 a carat in the 1-2 ct size, but increase
dramatically over 2 carats. Top spinels in the 2 to 5 carat size will often sell for $3,000 to $5,000
a carat. For the very rare stones over 5 carats, prices in the range of $8,000 to $10,000 per carat
are not unusual for the top colors.

Figure 11: Represents Spinel Market in Chanthaburi Thailand

Buyers await sales offers at the teeming Chanthaburi gem market, held every weekend. Many
buyers post handwritten signs detailing their wants at the front of the table or on the walls behind
them. Spinel: average retail prices, Jul 2020 in Bangkok gemstone world market Thailand.

Spinel is a gemstone that can be red, blue, purple, rose, black or brown in color, or also colorless.
Its red and blue varieties are sometimes confused with ruby or sapphire, two of the gemstone
variations of the mineral corundum, because of their striking visual resemblance. For that reason,
spinel had received insufficient recognition in the past. Now that consumers and gemstone
dealers are able to accurately identify spinel, it is starting to gain appreciation for its own
properties.
High-quality blue or red spinel gemstones are actually very rare - even rarer than sapphire or
ruby specimens of similar quality. Nevertheless, spinel's price, even in those cases, is still lower
than the price of sapphire and ruby. Color is an important factor that defines spinel's price - red
spinel is rarer than other varieties, and also more expensive; blue exemplars are the second rarest
spinel gems. Other varieties are more common and less sought after. Clarity is another important
factor that defines spinel's value - stones with no visible inclusions are more appreciated.

Spinel can be synthetized, but synthetic spinel's value is much lower than that of natural
specimens. Natural spinel's supply is somewhat limited, which contributes to its rarity.The
highest quality spinel often comes from Myanmar, with other mining sites in Afghanistan, Sri
Lanka, Vietnam, Cambodia, Tanzania, Madagascar, and others.

Figure 12: Represents Workers sorting SPINEL concentrate from the Baw Mar mine near
Mogok.
 Figure 13: Represents Talad Ploy exported gemstones worth $650 million in 2012

SPINELS RELATION WITH GREAT WORLD LEADERS

Pity the poor spinel

The gemstone that many people have never heard of has been saddled for centuries with the
name “the great impostor,” Alessandro Borruso said as he sipped a morning espresso in the cafe
of Sotheby’s London offices, where he is head of sales for Europe.

The spinel acquired that pejorative name for good reason. For hundreds of years it was believed
that spinels and rubies were the same stone. Indeed, a spinel can be every bit as beautiful as a
ruby, Mr. Borruso said, with a rich color that he called “stoplight red.”

And, according to Courtney Stewart, a senior research assistant in the Department of Islamic Art
at the Metropolitan Museum of Art, “rubies and spinels are known to be found in the same
mines, which would have further confused these two very similar looking stones.”

No wonder no one suspected there was a difference until 1783, when the French mineralogist
Jean-Baptiste Louis Romé de l’Isle developed a test that showed a spinel was not a ruby, said
Claibourne Poindexter, an associate jewelry specialist at Christie’s New York. And there was
still more bad news.

Figure 14: Represents Queen Elizabeth II in 2016 wearing the Imperial State Crown. A large
uncut red spinel, called Black Prince’s Ruby, is set into the front.
REFERENCES

https://www.gia.edu/gems-gemology/spring-2017-gemnews-fine-gems-update-myanmar

https://www.thehindu.com/thread/arts-culture-society/a-market-like-youve-never-
sheen/article19384978.ece

RUBIES AND SAPPHIRES FROM WINZA, CENTRAL TANZANIA Dietmar Schwarz,

Vincent Pardieu, John M. Saul, Karl Schmetzer, Brendan M. Laurs, Gaston Giuliani, Leo
Klemm, Anna-Kathrin Malsy, Eric Erel, Christoph Hauzenberger, Garry Du Toit, Anthony E.
Fallick, and Daniel Ohnenstetter

Abduriyim A., Kitawaki H. (2008) New geological origin: Ruby from Winza of Tanzania.
Gemmology, Vol. 39, Issue 8, No. 467, pp. 4–7 [in Japanese with English insert].

Appel P., Möller A., Schenk V. (1998) High-pressure granulite facies metamorphism in the Pan-
African Belt of eastern Tanzania: P-T-t evidence against granulite formation by continent
collision.

Journal of Metamorphic Geology, Vol. 16, pp. 491–509. Balmer W.

Leelawatanasuk T., Atichat W., Wathanakul P., Somboon C.(2006) Update on FTIR
characteristics of heated and unheated yellow sapphire.

https://www.gia.edu/doc/Winter-2008-Gems-Gemology-Rubies-Sapphire-Winza-Tanzania.pdf

https://www.gemrockauctions.com/learn/additional-gemstone-information/mahenge-spinel-the-
stone-that-changed-everything.