Characterization of recently discovered common green opals from Anosy (Madagascar)

The recently discovered common green opals from Anosy in Madagascar were studied for their physical, chemical and gemological properties and gemological relevance. The color of the opals is yellowish green but not very homogeneous for the presence of cavities, dark lamellae and spots, the diaphaneity is translucent / opaque with greasy luster and are inert to long and short wavelength UV radiation (366-254 nm). Refractive index and specific gravity values range between 1.435-1.460 and 2.03-2.07, respectively. The opals are CT type with tridymite more abundant than cristobalite and contain clay minerals (saponite). The main chromophores that determine the color are Fe and, subordinately, V and Cu. Other detected trace elements are Mg, Al, Ca, K, Na, Ni and Cr. The high contents of Ba, probably deriving from mica and feldspars, are noteworthy and can represent a geochemical marker. Three types of microstructures were observed: homogeneous and very fine microspheres, lepispheric cauliflower structures and globular aggregates of different sizes formed by laminae. Our analyses suggest that opals formed under similar genetic conditions, with a slow or moderate growth rate. Anosy opals would certainly have a good commercial value as semi-precious materials, although lower than those from other regions of Madagascar (Bemia).