№71-15

Study of colour change factors for some varieties of gems

S. Shevchenko¹, I. Tkachuk¹

¹ Dnipro University of Technology, Dnipro, Ukraine

Coll.res.pap.nat.min.univ. 2022, 71:170-178

https://doi.org/10.33271/crpnmu/71.170

Full text (PDF)

ABSTRACT

Purpose. To explain the main factors of color transformations on specific samples of precious stones with a change in color, as well as their synthetic analogues and imitations.

Methods. The work uses X-ray fluorescence analysis, as well as general scientific methods of research –empirical and theoretical (analysis, generalization, comparison, explanation). During the research, lamps with different light spectra (different color temperatures) were used: an incandescent lamp, a yellow diode lamp, a special gemological lamp.

Findings. The content and concentration of chromophore ions were determined for samples of precious stones and their imitations with color changes, namely alexandrite, chrysoberyl, sultanite, Malaya garnet, garnets with color changes, as well as synthetic corundum and alexite (glass). The influence of the concentration ratio of chromophore ions on the color of alexandrites and garnets with color change is shown. Color change for anisotropic stones with pleochroism is shown. The colors of anisotropic stones along the crystallographic axes were studied –two colors for uniaxial stones and three – for biaxial stones.

The originality. The change in color of the studied samples from the department's collection under the influence of four sources of illumination: daylight, an incandescent lamp, a yellow diode lamp, and a special gemological lamp was demonstrated. Chromatic charts are constructed with different color temperatures and wavelengths for each light source. It is shown that when the color temperature of light sources increases, the wavelengths of light in all samples, except for alexandrite, decrease.

Practical implementation. The data presented in the article demonstrate specific examples of the influence of factors on the change in the color of precious stones and provide an opportunity for additional explanations of this phenomenon for specialists, as well as expand its understanding by direct consumers of relevant services on the market. It is shown that with the gradual replacement of incandescent lamps with modern diode lamps, garnets with color changes and synthetic corundum "under alexandrite" will be in the most "losing" situation - their colors will be closer to each other and will hardly show strong changes. At the same time, such an artificial material as alexite (which is passed off as sultanite, in particular) will attract the attention of even uninformed buyers due to a sharp change in color, while the colors will almost match under such opposite sources as daylight and an incandescent lamp.

Keywords: gemstones with color change, chromophore ion concentration ratio, color temperature, Usambara effect, alexandrite, sultanite, alexite.

References

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