Original paper

A study on the characteristics of some C- and CT-opals from Brazil

Caucia, F. Ghisoli


Physical properties of blue and fire opals found in Piauí state, Brazil, have been characterized through optical analysis, specific gravity measurements, XRPD and IR spectroscopy, whereas chemical composition has been determined by LA-ICP-MS and SEM-EDS. Values obtained for refractive index and specific gravity are n = 1.430-1.461 and G = 1.98-2.28, in agreement with literature. The results of the XRPD and IR-spectroscopy show the cristobalite presence (C and -CT type). Trace quantities of chromophore elements are correlated with the variations of colour. The particular "coralloid islands" structure, built up by microspherules of amorphous silica, observed by SEM analyses could explain the particular effects of iridescence shown by some blue specimens.


gemsfire opalspectroscopic analysestrace elementsplay of colour