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

      Curtis, Neville J; Gascooke, Jason; Johnson, Martin R; Pring, Allan (MDPI, 2019-05-15)
      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 ...