Physical, geochemical and gemological properties of opals from Faroe islands

The opals from Faroe Islands are commercially not very relevant but are important from the historical point of view. Nice yellowish-red color fire opal and common opals from these islands are also very appreciated by collectors and can reach high prices in the markets. The analysed opals derive from Hoyvik near Torsshavn, in the island of Streymoy. Parent rocks are represented by the "middle basalt" formation. Investigated opals generally show vitreous-dull luster and specific gravity values comparable with data in literature. They are generally CT type with variable contents of cristobalite and tridymite and, in some cases, chalcedony. In order of abundance, trace elements are represented by Fe, Al, Ca, Mg, Na, P and K; Fe contents are very variable. The distribution of the elements is dictated expecially by cristallochemical substitution in the pseudo crystalline structure of the opals. A distinct feature of the opals from Faroe is represented by the presence of P; this element probably derives from metasomatic fluids. Trace elements probably derive from feldspars and pyroxenes in the host basaltic rocks. SEM investigations highlighted different textures like random piling of individual nanograins, lepispheres, tablets and plates that are typical of magmatic CT opals. The variable chemical composition and physical properties, and the different microtextures, would indicate the opals from Faroe Islands formed from different solutions, through variable growth rates.