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

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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.
Original languageEnglish
Article number299
Pages (from-to)1-20
Number of pages20
Issue number5
Publication statusPublished - May 2019

Bibliographical note

This is an open access article distributed under the Creative Commons Attribution License (CC BY) which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited


  • opal
  • hyalite
  • silica
  • X-ray diffraction
  • raman
  • infrared
  • 29si nuclear magnetic resonance
  • SEM
  • Provenance


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