Gemmological Characteristics and Play of Colour Effect of Artificial Opals from Russia

The FTIR, EDXRF, ESEM and TG-DTG testing methods are employed in this paper to investigate gemmological characteristics, the coupling relationship between the diameter of the SiO₂ sphere, the interspace of the spheres, the compositions of the cementing substance and play of colour effect in the artificial opals from Rusgems Company of Russia, including the zirconia-type and resin-type artificial opals. The results show that the zirconia-type artificial opal is similar to the natural opal in the chemical composition, mechanical character and structure, but only a few of zirconia present in the cementing substance. The IR absorption band at 1730 cm^-1 is attributed to the ν(C=O) stretching vibration of acrylic resin, and the weak IR absorption band at 1432 cm^-1 is attributed to the δ(CH₂) curve vibration in the resin-type artificial opal. The small SiO₂ spheres with the average diameter of 215 nm are accumulated closely as hexagon or cube in the three-dimensional space in the resin-type artificial opal. The interspaces of the spheres approaching equal in size are cemented by the artificial resins, where the octahedral and tetrahedral spacings are presented. The play of colour effect is controlled directly by the diameter size of the SiO₂ sphere, the type and the interspace of the spacing.