However, although these gem 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, its research partners of the Universities Antananarivo, Madagascar, Dar es Salaam, Tanzania, Nairobi, Kenya and Glasgow, UK,  with the companies Swala Gem Traders at Arusha, Tanzania, tGemmological Institute of America, Bangkok, Thailand, mining company ORYX, Paris and the CNRS, have drawn up the first identification card for green garnets, according to their deposit of origin, providing a first step towards certification of the new gemstone and greater added value on the jewellery market.

Gemstones from a vanished 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, (later murdered for his mining  concessions.) Green garnet exists almost exclusively in Tanzania, Kenya, Madagascar, Pakistan and Antarctica.

Deposits are uniqueand locations are well known, but determining the each stone’s country of provenance is not easy. 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 600m years when supercontinents East 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 of clayey and organic materials accumulated on the sea floor. These were rich in elements, particularly vanadium and chromium –which give the garnet crystals their green colour.

In the convergence process which drove the closure of this ocean, the East Gondwana plate plunged beneath its West Gondwana counterpart, dragging the 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 forms, graphitic gneiss. It is why tsavorite is now found exclusively in gneiss formations, as nodular concretions of 5 to 20 cm diameter.

The origin of each garnet retraced
In these circumstances it is difficult 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 composition, 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. On the strength ofprevious discoveries about other gemstones, researchers applied this isotope analysis method to green garnet. They 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 is attributed to each deposit, expressed as per mil (‰). For example, tsavorites originating from the North of Tanzania show the highest ratios, ranging from 15-21‰, those coming from South of the country give low values,  9-11‰. The Kenyan, Madagascan, Pakistani and Antarctic garnets have an intermediate isotope ratio, between 11-15‰.

Next is their colour, along with  vanadium, chromium and manganese concentrations, which give finer distinction between the garnets of each of these countries. The geologists have set up a new register of identity database as an aid to 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. Results of these investigations provide key information to locate the geographical origin of each stone. They also represent a first step towards certification for green garnets.

The procedure is proving essential for accurately setting the trade value of these gemstones and giving them high added value on the jewellery market.