Volume 24, Number 2, April - June 2010
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(Follow this link for abstracts of past issues)
Microstructures observed in Andamooka matrix opal
Grant Pearson
ABSTRACT
Stereomicroscopic inspection and photomicrography of various specimens of Andamooka matrix opal revealed a range of microstructures, which differed from its usually presumed constitution of a simple quartz-grained sandstone cemented together by opal. The most prevalently observed microstructure indicated that much of this matrix variety of opal may be a pseudomorph after a pre-existing massive cryptocrystalline formation of a non-orthogonal mineral such as calcite, and much less probably of an orthogonal/cubic precursor, such as the evaporite mineral, halite. Evidence suggestive of oolitic origin and oven of biogenic marine or estuarine derived structure was also observed.
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Assorted rough matrix opal from Andamooka. |
Stained and polished Andamooka matrix
opal. |
The long-known and exploited porosity of this material appears to consist of the remnant interstices and voids between the microcrystals of the original mineral that have not been completely infiltrated by the replacing opaline silica. This porosity feature enables matrix opal to be stained by infiltration of a pigmenting black medium to develop a gem material resembling the famous black opal of Lightning Ridge, New South Wales.
Keywords: Opal, diffraction, play of colour, microcrystal, matrix, Andamooka, sandstone, pseudomorph, calcite, interstices, voids, staining, oolite, ooids.
parcel
A study of prismatine from Begogo, Madagascar
Dr Valentina Bordoni, Dr Massirno Bolocchi and Prof Franca Caucia
ABSTRACT
The uncommon mineral kornerupine has been studied: it is a rare orthorhombic Mg and Fe alumino-boro-silicate ideally (Mg,Fe2+)3Al6(Si,Al,B)5O21(OH); the studied samples come from Begogo (Midongy massif, south-east of Madagascar). Crystals of dark green kornerupine have been characterized as a prismatine species by means of X-ray single crystal study and
LA-ICP-MS microanalysis (results of these analytical investigations are not presented in this work). Standard gemmological tests have been performed on two faceted samples of different colour. All the results have been compared with literature data for kornerupine from other worldwide occurrences.
Key Words: prismatine; Begogo, Midongy massif, Madagascar; chromophore elements; zircon micro inclusions; gem quality mineral.
Coated and fracture-filled coloured diamond
Song Zhonghua, Lu Taijin, Shen Meidong, Su Jun, Dong Jing, Zhang Xihuan|
ABSTRACT Gemmological characteristics of both colour and clarity enhancements have been found in a coloured diamond. Recently, we examined a brownish orange-yellow diamond, which displayed combined features of coating and fracture filling. Magnification revealed typical damage to the coating layer and a 'flash-effect' from filled fractures. EDXRF analyses suggested that Ag, Fe, Ti, AI, Pb, Br are the major trace elements of the coating. The diamond was treated by Pb and Br glass filling, and followed by coating involving Ag and Fe nanoparticles. Keywords: coated diamond, fracture filling, nanoparticles. |
 Many orange-yellow spot, scratch lines and abrasion at the facet junctions on the pavilion of this 2.05 ct round brilliant diamond are indicative of coating. |
A chatoyant malachite
Gagan Choudhary|
ABSTRACT Recently, the Gem Testing Laboratory of Jaipur, India encountered an interesting specimen of deep green malachite weighing 9.57 carats, which displayed a broad chatoyant band. This report describes this unusual and interesting specimen of chatoyant malachite. Keywords: Malachite, chatoyancy, fibrous structure, IR Spectrum. |
 This 9.57 ct saturated green malachite displayed a broad chatoyant band. Also note the fine bands along the length of the cabochon. Because of the challenges of photography, a good image could not be produced. |
Cultji Diamond Selector IIITM
Instrument Evaluation Committee Report Trevor Linton and Anne Paul
ABSTRACT
Diamond probes are not new and many gemmologists may argue that if a defining instrument is to be purchased then one that separates diamond from synthetic moissanite would be preferred. However when one compares the price of a combined testing instrument from the cost of a singular diamond probe plus a hand lens, there is a notable differential.
Recent advancements in the electronic design of diamond probes incorporate locality controlled testing of gemstones as either air conditioned or hot weather conditions, as the differential between conditions and locality will produce errors into tests such as corundum indicating diamond. Unfortunately this problem plagues all diamond and dual diamond/moissanite probes.
Key words: Diamond testers, Diamond probes, Thermal Inertia probes.
