Prase Opal from Tanzania

Recently there appears a new gem from Tanzania in the market,which is called prase opal.The colour of prase opal is bright apple-green and it rises people's concern.This paper mainly analyzes the gemmological and mineral characteristics of prase opal form Tanzania.With the help of electron microprobe analysis (EPMA), X-ray power diffraction(XRD) X-ray fluorescence spectrometer (XRF), infrared spectroscopy and ultraviolet-visible-near infrared spectroscopy, the mineral components, trace elements, vibrational spectrum and colour mechanism of prase opal are analyzed, The results indicate that the main mineral components of prase opal are amorphous silica and a certain amount of low-tridymite, and low-cristobalite. The host rock of prase opal consists of amorphous silica, goethite and low-tridymite, low-cristobalite, with also a small number of magnetite, chromite and psilomelane distributing sporadically. Moreover the host rock has been weathered seriously,which shows earthy luster. X-ray fluorescence spectrometer results indicate that the main element of prase opal is Si, and there are also some transition elements such as Ni and Fe with also a bit of Cr and Cu. Because of the existence of water, the loss on ignition of prase opal are approximate 6%. It also shows that the main elements of host rock are Si and Fe with some amount of Mg,Al, Cr, Ca and Ni,which are different from those of the prase opal. Compared with the element contents of prase opal, the host rock contains more Fe, Mg, Al, Ca, Cr and less Ni. Moreover, the LOI of host rock is more, about 7%. The infrared spectrum of the prase opal proves that the samples are opal not chalcedony again in spite of partly weak micro-fibrous phenomenon observed by optical microscope. UV-Vis-NIR spectrum shows broad bands at 400,650 nm in the visible region. It also shows some bands which caused by various existent form of water. Prase opal contains trace elements including Ni and Fe. The octahedrally coordinated divalent nickel cause three clearly typical d-d bands, and two bands which from ³A_2g(F) → ³T_1g(F)(389 nm)and ³A_2g(F) → ³T_1g(P) (654 nm)are responsible for the green colour of the prase opal. Moreover, the presence of Fe³⁺ modify the colour to be more yellowish green.