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Abstract:The mineral and fluid inclusions trapped by gemstones during the metamorphism of rocks in carbonate platform successions are precious markers for the understanding of the gem genesis. The nature and chemical composition of inclusions highlight the major contribution of evaporites through dissolution or fusion, depending on the temperature of formation from the greenschist to the granulite facies. The fluids are highly saline NaCl-brines circulating either in an open system in the greenschist facies (Colombian and Afghan emeralds) and with huge fluid-rock metasomatic interactions, or sulphurous fluids (ruby, garnet tsavorite, zoisite tanzanite and lapis-lazuli) or molten salts formed in a closed system with a low fluid mobility (ruby in marble) in the conditions of the amphibolite to granulite facies. These chloride-fluoride-sulphate ± carbonate-rich fluids scavenged the metals essential for the gem formation. At high temperature, anions SO42-, NO3-, BO3-and F-which are powerful fluxes lowered the temperature of chlorine and fluoride ionic liquids. They provided transport over a very short distance of aluminium, and/or silica and transition metals which are necessary for gem growth. So, the genetic models proposed for these high-value and ornamental gems underline the importance of the metamorphism of evaporites formed on carbonate continental shelves, and emphasises the chemical power accompanying the metamorphism at moderate to high temperatures of these particular evaporite-rich and organic matter-rich protoliths to form gem minerals.Suggested Reviewers: Bernd Lehmann, PhD. Powered by Edit orial Manager® and ProduXion Manager® from Aries Syst em s Corporat ionEditor Mineralium Deposita, Technical University of Clasthal lehmann@min.tu-clausthal.de Dr. Bernd Lehmann, Editor of Mineralium Deposita, who invited us to write this paper on the role of evaporites in the formation of metamorphic gems, published in a short version as "Le fluide, l'Arlésienne du métamorphisme" in the Journal Géochronique, N°136, décembre 2015 entitled "Regards croisés sur le métamorphisme".

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