ArticlePDF Available

Internal Structures and Identification of Gilson Synthetic Opals

Authors:
  • The CIBJO Academy

Abstract

The French chemist, Pierre Gilson , has been synthesizing gemstones for some years now, and we have recently had the opportunity to examine a number of his synthetic opals. His latest opals, both black and white, were so convincing-almost too good to be true-that we felt it desirable th at some of their internal structural features should be more generally known. Our survey includes Gilson synthetic opal cabochons obtained in London from early 1974 until December 1975.
A preview of the PDF is not available
... Although the opal-formation process may be approximated by using synthetic monodisperse silica spheres, the structure only approximates opal-AG. Columnar grains are often observed in synthetic opal (Jobbins et al. 1976), whereas these vertically elongated grains are not observed in natural opal-AG, probably a direct result of the higher rates of sedimentation in synthetic samples. Also, the water in synthetic opals is exclusively internal silanol groups, i.e. no molecular water is present (Graetsch and Ibel 1997). ...
Thesis
Full-text available
Australia is the world’s largest producer of precious opals, contributing more than $1 billion per annum to the GDP. However, to date little fundamental research has been carried out on banded opals, which are potentially the most valuable of all opal varieties. Opal is also Australia’s national gemstone; yet for such an important resource, it is surprising that the mechanisms of opal formation remain in dispute, in particular for banded opals. The focus of this study is to understand the formation of opal by investigating the chemistry and microstructure of banded and non-banded opals. Opals from several regions of Australia (Coober Pedy, Lightning Ridge, Andamooka and Tintenbar), in addition to opals from Mexico, were thus investigated in detail using a range of techniques. Evaluation of the trace element chemistry of opals was carried out by employing a combination of experimental techniques, including Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICPMS) and Secondary Ion Mass Spectrometry (SIMS). Darker coloured bands contained significantly higher concentrations of certain transition elements (Ti, V, Co, Ni, Cu, Zn and Y) and rare earth elements (La, Ce) than the lighter coloured bands. The concentrations of other elements (Mg, Ca, Al, Fe and Mn) were in most cases found to be similar between bands. Some elements (Ti, Cr, Cu, Zn, Co and Zr) were found to be distributed more heterogeneously than others (Na, Ca, Mg, K, Al and Fe). Based on this evidence, a solution depletion model was proposed to explain the formation of banded opals, involving the charge neutralisation of silica colloids by highly charged transition metal cations. The microstructural characteristics of several Australian opal-AG (amorphous, gel-like opal) specimens were studied using a number of experimental techniques such as porosity measurement, thermomechanical analysis (TMA) and thermogravimetric analysis (TGA). An initial expansion followed by contraction was observed in TMA. The temperature at which this ‘transition’ occurred ranged from 200 to 400˚C and was found to be region dependent. TGA revealed that the temperature range, from 215 to 350˚C, over which the maximum rate of dehydration occurred, was again region dependent, consistent with the TMA data. A dehydroxylation–sintering mechanism was proposed to account for these results. Porosity measurement yielded a greater degree of porosity in the opaque white samples than the transparent ones; the additional voids consequently scatter light internally, rendering the opal opaque. 29Si NMR and 27Al NMR experiments were undertaken to characterise the relative disorder, silanol content and the coordination state of Al within opal-AG and opal-CT (cristobalite-tridymite opal). The comparison of 29Si NMR spectra demonstrated that opal-CT samples contained a higher proportion of both geminal (Q2) and vicinal silanol groups (Q3) than opal AG. This result was attributed to the large internal surface area of opal-CT compared to that of opal-AG. Since Al was found to exist in a tetrahedral coordination within the opal structure, incorporation of Al occurred through substitution of Si during the period of colloidal growth. As the concentration of Al in banded opals was similar, the colloids within each band are considered to have formed at similar times.
In Deuelot nomts and f1i~hli~hl' at C IA', Lab in N et» York
  • I Crown Inqshield
I. Crown inqshield, R. 1973. In Deuelot nomts and f1i~hli~hl' at C IA', Lab in N et» York. Gems Ge moI., XIV, (6). 172·17 9.
In Developments and H ighlights at U / A's La h ill Los Angeles
  • R T Lidd Icoat
Lidd icoat, R. T. 1974. In Developments and H ighlights at U / A's La h ill Los Angeles, Ge ms Gc rnol., XIV, (10), 309·3 16.
In Dcvtlopmmt s and Highlitht s ti t C IA ' of Lab In LOJ Angeles. Ge ms Gcm ol
  • R T Lid Dicoat
Lid dicoat. R. T. 1974. In Dcvtlopmmt s and Highlitht s ti t C IA ' of Lab In LOJ Angeles. Ge ms Gcm ol...
Note s on Gitson synthetic wlllu opal (S eptember, 19 75 ) . .l-Ge rnm
  • K Scnrrn U
Scnrrn u, K. 1976. Note s on Gitson synthetic wlllu opal (S eptember, 19 75 )..l-Ge rnm.. XV, (2), 62-65.
NMtJ on 111, ldmt ificoticn of Gilson Synthetic Opals
Tom bs. G. A. 1975..NMtJ on 111, ldmt ificoticn of Gilson Synthetic Opals. Aust. Gc m., Xli, (6), 179.1 80.
  • G A Tom Bs
Tom bs. G. A. 1975..NMtJ on 111, ldmt ificoticn of Gilson Synthetic Opals. Aust. Gc m., Xli, (6), 179.1 80.