A Review of the Classification of Opal with Reference to Recent New Localities

Abstract

Our examination of over 230 worldwide opal samples shows that X-ray diffraction (XRD) remains the best primary method for delineation and classification of opal-A, opal-CT and opal-C, though we found that mid-range infra-red spectroscopy provides an acceptable alternative. Raman, infra-red and nuclear magnetic resonance spectroscopy may also provide additional information to assist in classification and provenance. The corpus of results indicated that the opal-CT group covers a range of structural states and will benefit from further multi-technique analysis. At the one end are the opal-CTs that provide a simple XRD pattern (“simple” opal-CT) that includes Ethiopian play-of-colour samples, which are not opal-A. At the other end of the range are those opal-CTs that give a complex XRD pattern (“complex” opal-CT). The majority of opal-CT samples fall at this end of the range, though some show play-of-colour. Raman spectra provide some correlation. Specimens from new opal finds were examined. Those from Ethiopia, Kazakhstan, Madagascar, Peru, Tanzania and Turkey all proved to be opal-CT. Of the three specimens examined from Indonesian localities, one proved to be opal-A, while a second sample and the play-of-colour opal from West Java was a “simple” Opal-CT. Evidence for two transitional types having characteristics of opal-A and opal-CT, and “simple” opal-CT and opal-C are presented.

Crystallinity and Play-of-Colour in Gem Opal with Digit Patterns from Wegel Tena, Ethiopia

“…Wilson explained the opal-CT structure by a tridymite-dominant paracrystalline model [3]. Recent work by Curtis et al [4] found that opal-CT appears to be a paracrystalline form of silica that has some structural characteristics of cristobalite and tridymite.…”

“…Other techniques, such as infrared spectroscopy [5][6][7] and Raman spectroscopy [8][9][10][11][12][13][14], have also been used to distinguish different types of opal. Furthermore, Raman spectroscopy has been considered a method to distinguish the complex structural relationships between different types of opal [4,8,10,13].…”

FAAS, FT-IR and XRD Identification of Natural and Heat Treated Opals Located in Wadla Woreda, North Wello, Ethiopia

“…X-ray powder diffraction data were obtained using powder diffractometer working with graphite monocromatized Cu Kα radiation in the range 2θ, 10-70°. The X-ray powder diffraction analyses performed are able to classify opals in to their common groups such as opal-A, opal-C and opal-CT [25,29]. XRD patterns for natural and heat treated opals are shown in (Fig.…”

Raman Spectroscopy Study of Phosphorites Combined with PCA-HCA and OPLS-DA Models

“…The common methods for determining phosphorite grade include conventional chemical analyses, infrared spectroscopy, nuclear magnetic resonance, X-ray spectroscopy, and plasma mass spectrometry, etc. [5][6][7][8][9][10], however, most of these methods require complex sample preparation and are difficult to perform on site for underground measurements. Therefore, it is of great interest to study a powerful method to meet these difficulties.…”