Actualité scientifique
Scientific news
Green garnet1, recently
introduced on to the
gemstone market, is
renowned for its brilliance,
hardness and rarity –much
rarer than diamond for
example. On top of these
fine qualities this precious
stone has a high level of
purity and a lower price
compared with emerald,
its direct competitor since
it has the same colour.
These properties give it
high economic potential
for the producer countries,
essentially Tanzania and
Kenya.
However, although these
gemmological characteristics2 have already been
described and are now
well known to experts, its
genesis and mineralogical
and geochemical
properties, which would
be useful for determining
its geological and geographical origin, have not up
to now been clarified. An
IRD team and its research
partners3 have drawn up
the first identitification card
for green garnets
according to their deposit
of origin. A first step
towards certification of the
new gemstone and greater
added value on the
jewellery market.
© GIA / V. Pardieu
April 2011
Towards certification
of East-African green garnet
© IRD / G. Giuliani
N° 371
Actualidad cientifica
Green garnet is extracted in small mines found almost exclusively in Tanzania and Kenya, and also Madagascar.
Green garnet 1, newly arriving on the jewellery
market, with the colour of emerald, its brilliance and
hardness, make it a serious rival for other precious
stones. However, if its international commercial
potential is to be increased, this gemstone2 needs to
acquire its certification. In other words, a way must
be found of accurately tracing the geographical
origin of each stone. This has now been realized by
an IRD scientific team and their research partners3.
Gemstones derived
from a disappeared ocean
Under its trade name “tsavorite”, called after the
Kenyan «Tsavo-National Park» reserve where it was
discovered in 1971 by the Scottish geologist Campbell R. Bridges, green garnet exists almost exclusively in Tanzania, Kenya, Madagascar, Pakistan and
Antarctica4. These deposits are unique in the world
and locations are well known, determining the each
stone’s country of provenance is not easy for scientists. It is exactly because they belong to just one
and the same geological entity, extending from
North-East Africa to Antarctica, that it is difficult to
distinguish them. This ensemble was formed 600
million years B.P. (before present) when the two
supercontinents East Gondwana and West Gondwana were converging. Before the present-day East
Africa rose up owing to plate-tectonic movements, a
sea, the Mozambique Ocean, separated these
continents. Sediments consisting of clayey and
organic materials had accumulated on the sea floor.
These were rich in elements, particularly vanadium
and chromium –which eventually give the garnet
crystals their green colour. In the process of convergence which drove the closure of this ocean, the
East Gondwana plate plunged beneath its West
Gondwana counterpart, dragging these sediments
with it down towards the deep reaches of the continental crust. High pressures and temperatures of
between 600 and 750°C that the tectonic movements and burial processes generated then crystallized the garnets and transformed the sedimentary
For further information
rocks into metamorphic forms , graphitic gneiss.
This is why tsavorite is now found exclusively in
these gneiss formations, as nodular concretions of
5 to 20 cm diameter.
5
The origin of each garnet retraced
In these highly exceptional circumstances it is difficult, although nevertheless common to all green
garnets, to distinguish a Tanzanian stone from a
Kenyan or even a Madagascan one. However, it is
known that the properties of each deposit depend
on those of the rock in which gemstone crystallized,
the mother-rock. Consequently the isotopic composition6, especially concerning the oxygen contained
in the crystals*, acts as a good marker of the environment where they formed. Study of this property
has already proved its effectiveness for retracing
emeralds, rubies and sapphires to their origins. Now
it is green garnet’s turn to reveal its secrets.
On the strength of their previous discoveries about
other gemstones, the researchers applied this
isotope analysis method to green garnet. They
therefore collected samples from 24 deposits,
situated in each of five countries where the mineral
is mined, and analysed the ratio between the two
isotopes of oxygen, 18O and 16O. A characteristic
value of this ratio has been attributed to each
deposit, expressed as per mil (‰). For example,
tsavorites originating from the North of Tanzania
show the highest ratios, ranging from 15 to 21‰,
whereas those coming from the South of the country
give low values, only 9 to 11‰. The Kenyan, Madagascan, Pakistani and Antarctic garnets have an
intermediate isotope ratio, between 11 and 15‰.
Next it is their colour, along with their vanadium,
chromium and manganese concentrations, which
give a finer distinction between the garnets of each
of these countries. The geologists have set up a
new database, a register of identity as an aid for
finding each garnet’s provenance according to its
composition.
Contact
Gaston GIULIANI,
researcher at the IRD
Tel: +33 (0)3 83 59 42 38
giuliani@crpg.cnrs-nancy.fr
UMR géosciences environnement
Toulouse - GET (IRD / Université Paul
Sabatier - Toulouse 3/ CNRS)
Address
CRPG-CNRS
15 r. Notre Dame des Pauvres
BP 20
54501 Vandœuvre-lès-Nancy
Referencies
gaston giuliani, fallick a.e., feneyrol j.,
ohnenstetter d., pardieu v., saul m.
18 16
O/ O and V/Cr ratios in gem tsavorites
from the Neoproterozoic Mozambique
Metamorphic Belt: a clue towards their
origins? Mineralium Deposita
For the East-African countries which mine their
tsavorite deposits, this study gives the opportunity to
establish exploration guides for prospecting for new
deposits. The results of these investigations provide
key information for locating the geographical origin
of each stone. They also represent a first step
towards cer tification for green garnets. This
procedure is proving essential for accurately setting
the trade value of these gemstones and giving them
high added value on the jewellery market.
feneyrol j., giuliani gaston, ohnenstetter d.,
le goff e., malisa e. p. j., saul m.,
saul e., saul j., pardieu v. Lithostratigraphic and structural controls of ‘tsavorite’
deposits at Lemshuku, Merelani area,
Tanzania. Comptes rendus geoscience,
2010, 342 (10), p. 778-785.
Copy editor – Gaëlle Courcoux - DIC, IRD
Translation – Nicholas FLAY
Key words
Green garnet, tsavorite, East Africa,
origin, certification
1. A silicate of calcium, aluminium, vanadium and chromium.
2. Related to gemstones: fine, precious or ornamental stones meeting criteria for colour, brilliance, hardness and rarity.
3. These investigations were conducted in partnership with the Universities of Antananarivo in Madagascar, Dar es Salaam in Tanzania,
Nairobi in Kenya and Glasgow in the United Kingdom and also with the company Swala Gem Traders at Arusha in Tanzania, the
Gemmological Institute of America in Bangkok in Thailand, the mining company ORYX in Paris and the CNRS.
4. Along the Mozambique Metamorphic Belt.
5. That is formed by the recrystallization of sedimentary rocks through the action of very high pressures and temperatures.
*Did you know ?
Oxygen is the main constituent of precious stones,
except diamond. As proportions by weight, it makes
up 45% for emerald, 47% for ruby and sapphire and
42% for green garnet.
1
2
3
1 000 million years ago
700 million years ago
600 million years ago
East
West
Sediments
Eastern
Gondwana
Mozambique Ocean
Oceanic crust
Continental
crust
Western
Gondwana
st
ni
ea
Oc
Terrestrial crust
ru
cc
Tectonic
plate
convergence
Terrestrial crust
Metamorphism with
crystallisation of
the green garnet
Terrestrial crust
Garnets are formed through metamorphism, a phenomenon induced by great rises in temperature and pressure generated by tectonic movements and burial
processes during lithospheric plate convergence.
© IRD / L. Corsini d’après schéma de Gaston Giuliani
© CRPG-CNRS / J. Feneyrol
6. Composition of different isotopes of chemical elements present in the stone. In fact, every element, like oxygen, can have several
isotopes which differ in their number of neutrons, the particles which give the atom its electric charge.
Coordination
Gaëlle Courcoux
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