Abstract: This article describes the examination of natural, synthetic and plastic simulated opals by basic gemmological testing, X-ray fluorescence analysis, infrared absorption spectra and thermal analysis. Specific gravity, 5 851, 5 730, 4 700, 1 730 cm^-1 infrared absorption spectra and trace elements CI, Zr etc are main characteristics for identifying natural opal and various synthetic opals. T he characteristics mentioned above also prove that Gilson opal is far lower than natural opal in water content.