GemResearch Swisslab (GRS) Specializing in Origin Determination of Fine Rubies and Sapphires

UV-VIS-NIR Absorption Spectroscopy of Natural Pastel and Vivid Colored "Padparadscha" - Sapphires Enhanced by the New E(IM)-Method.
Detection of Unstable Color Centers.

BOX: UV-VIS-NIR Absorption Spectroscopy
Spectroscopic analysis was carried out with a multi-channel spectrometer with 1024 diode arrays and an 80Watt Xenon lamp from J&M GmbH in Aalen, Germany. Quartz fiberoptics of 2mm diameter, and quartz lens optics, were used to condense the signal to the sample, with another quartz lens optic utilized to collect the signal. Before the sample, and after the condenser optic, a diffuser was implemented, then, after the signal, and before the second quartz lens optics, a UV-VIS polarizing filter was used to obtain polarized absorption spectra. This set-up enabled the spectra to be recorded from 260nm to 1024nm. Below 300nm, an increase in background noise was present due to the characteristics of the Xenon lamp and the increase in absorption of the UV filter towards the lower UV region. The resolution of the absorption lines is restricted by the number of diodes used to approx. 1nm, as lines narrower than this constraint could not be seen. The measuring time was 10 milliseconds. 20 spectra were collected in one run and averaged to obtain one spectrum. The data was acquired by software from the manufacturer (TIDAS).
The Zero absorption line was collected for each set up which allows correcting for the absorption characteristics of the polarizing filter before every measurement. Due to the nature of multi-channel spectroscopy, fluorescent emission lines (as seen, for example, as a white fluorescent line in a hand-held spectroscope) will also appear in an absorption spectrum as a negative peak towards lower absorption (See fluorescent lines in our spectra in the region 690-692). Their intensity is related to the Cr-concentrations. The application of multi-channel spectroscopy was used for the color measurement of gemstones (Lit 21). Polarized absorption spectra were found to be well suitable for testing rubies and sapphires in the GRS laboratory (“finger-printing”) and to characterize the types of colors.
Results of these analyses are given in Fig. 5, 22a, 24, 26, 27, 28, 29, 30, 31, 34 and 35.

Fig. 28 Polarized UV-VIS-NIR absorption spectra of a vivid orange sapphire (Origin: Songea, Tanzania) enhanced with the new method E(IM). Absorption parallel and perpendicular to the c-axis are shown. Note: Strong effect of pleochroism to the development of the Cr³⁺-band centered at 550 to 560nm. Prominent 450nm line in comparison to Fig. 27 and 29 are due to higher iron (Fe³⁺-pairs) concentrations. Chemical compositions See Table 7c. Absorption trends in the blue region of the spectrum are caused by color centers (See Lit 04).

Fig. 29 Polarized UV-VIS-NIR absorption spectra of a pastel orange-pink sapphire enhanced with the new E(IM1) method. Absorption parallel and perpendicular to the c-axis are shown. LA-ICP-MS data see sample GRS 12598 (Table 4). Cause of color dominated by isolated Cr³⁺ (band at 405 to 410nm and 550nm to 560nm) and Color centers and subordinate Fe(³⁺)-pairs (subordinate weak 450nm line) and subordinate isolated Fe³⁺ (weak 388nm line). Left side: Color shift after long term UV- exposure (shift to higher absorption in the blue region of the spectrum). Two absorption curves due to the presence of different types of color centers are indicated. Mg²⁺ -trapped hole color centers are interpreted causing the general increasing absorption in the blue region of the spectrum. They are stable (See blue curve at lower absorptions) (Compare Lit. 04). Upper yellow curve recorded after UV- exposure. This spectral shift is caused by a different color center which is unstable.