Rubies and sapphires belong to the corundum mineralogical family. Corundums exist in a wide range of colours. They consist of aluminium oxide crystals containing impurities which dote them with their various tints: titanium and iron for the blue of the sapphire, chromium and vanadium for the red of rubies. Humans have been captivated by the beauty of these precious stones for many centuries, yet the original environment of formation of some of them is still a matter for debate. This is especially the case for sapphires found in alkaline basalts, volcanic rocks from which most of the world’s commercialized blue sapphires are extracted. It is the extremely high pressures and temperatures prevailing several tens of kilometres deep in the Earth’s crust which generated corundums. Rising magma then brought them up to the surface where the crystals accumulated following the erosion of the surrounding protective rock. Prospectors then find these stones in placers which correspond to sedimentary deposits. That explains why it is extremely difficult to determine the origin of these stones from secondary deposits.
Geologists have nevertheless been attempting for several years to go back up to the primary genesis of corundums, basing their search particularly on the isotope composition of the oxygen (18O/16O) trapped inside these crystals. Compilation of the results of several international teams enabled them to establish a databank of isotopic values for oxygen for the whole of the world’s deposits of sapphires and rubies found in alkaline basalts. Yet although this parameter enables scientists to make progress towards revealing the genesis of these stones, geological study of a primary deposit is crucial for identifying unambiguously their provenance. In Madagascar, researchers at the University of Antananarivo, from the IRD and the CNRS recently gained access to a well preserved part of ruby bearing rock brought up by magma of mantle origin. This discovery represents the link that geologists needed in order to confirm the nature of the host-rocks of rubies, and also of sapphires, found in the alkaline basalts. The study of samples taken from the site were successful in indicating the conditions in which these rubies were formed: extremely high pressure of 20 kbar and a temperature of around 1100°C, pointing to a depth of 60 km, were necessary for these precious stones to generate. By combining this new field data with oxygen isotope compositions determined for 150 sapphires from basaltic placers originating from 13 different countries, the geologists succeeded in identifying the precise source of all rubies and sapphires found in alkaline basalts.
In the great majority of cases, the cross-referencing and combining of all these results led to confirmation of the magmatic origin for the sapphires found in these rocks. This result corroborates those from previous studies focusing on the chemical composition of various glasses trapped by these sapphires and which are typical of magmatic environments. Moreover, the existence of sapphires bearing syenite xenoliths (3) confirmed that these corundums were crystallized from a magma whose source was the mantle.
An unequivocal metamorphic origin was also determined for 20 % of the sapphires and for all the rubies of basaltic origin, 62 different samples coming from deposits in Asia, Australia and Madagascar. In this second scenario, the parent-rock no longer originated from the mantle but from the deep continental crust in the transition zone between the crust and the mantle. This type of primary deposit is encountered in high pressure and temperature environments which form sizeable outcrops in the ancient basements as in Madagascar.
Understanding the genesis of the rubies and sapphires found in sedimentary deposits could therefore help in the determination of their geological origin and thus increase the possibilities for mining these gemstones. Downstream of the extraction stage, the process could also be envisaged as a method for controlling the trading circuits. However, unlike emeralds, for which this type of study comparing field analyses and isotopic measurements provides the elements for identifying both the geological origin and the geographic location of the primary deposit, sapphires can reveal only their geological origin. A peculiarity which will probably leave part of the mystery cloaking these fascinating stones still intact for many years to come.
(1) This research was conducted by the ‘Laboratoire des Mécanismes de Transfert en Géologie (LMTG)’ of Toulouse and the ‘Centre de Recherches Pétrographiques et Géochimiques (CRPG)’ in Nancy jointly with the Scottish Universities Environmental Research Centre of Glasgow (Scotland), the Institut Gubelin in Switzerland, the Pakistan Geological Service and the universities of Hanoi (Viet-Nam) and of Antananarivo (Madagascar)
(2) Secondary deposits of precious stones formed by the accumulation of fluvial or marine alluvium.
(3) Syenite is a rock composed of more than 60% of potassium feldspars
Journal
Mineralium Deposita