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.