Gemological, physical and chemical properties of prase opals from Hanety Hill (Tanzania)

Abstract

Chemical, physical and gemological properties of some green “prase” opals from Hanety Hill in Central Tanzania were investigated. The color of the opals ranges from clear green to apple green, diaphaneity from translucent to semitransparent, luster is vitreous, and all result inert to UV lamp radiations. Specific gravity values are between 2.11-2.13, refraction indices between 1.439 and 1.458, comparable with literature data. XRD and FTIR analyses show the opals belong to the CT type, with tridymite higher than cristobalite. SEM observations revealed a lepispheric and mammillary structure formed by spherules with a diameter around 10-12 μm, on their turn composed of amorphous silica microspheres. The opals show a homogeneous chemical composition with very high contents of Ni and lower of Mg, Ca, Fe and transition elements like Zn, Cr and Co. Al and K are nearly absent. The chemical composition of the opals reflects that of serpentine rocks. Because of its abundance, the cromophore element responsible for the green color is Ni. Prase opals may have formed through the process of low temperature metamorphism, which generated the serpentinite from a magmatic ultrafemic rock.