Green garnet, 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 gemological characteristics 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 identification 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.
Green garnet1, 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 former 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 rocks into metamorphic forms5, graphitic gneiss. This is why tsavorite is now found exclusively in these gneiss formations, as nodular concretions of 5 to 20 cm diameter.
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.
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 certification 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.
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.
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.
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