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

Origin of Color: Natural Unheated (N) Violet-Blue Sapphires and Orange Sapphires Produced with the New E(IM)-Method


Fig. 34 Polarized UV-VIS-NIR absorption spectra of two sapphires with different colors. An orange-red sapphire enhanced by the new E(IM) method, and a violet-blue sapphire unenhanced by heat (N). For comparison: Both stones have iron (Fe), chromium (Cr), and titanium (Ti) concentrations, but the violet- blue colors are dominated by Fe²⁺ - Ti⁴⁺ charge transfer (band centered at 565nm) and Cr³⁺ - absorptions (broad band at 405-410nm and 554nm), while the orange-red colors are caused by isolated Fe³⁺ (388nm) and Fe^{3+ -pairs (377nm and 450nm) and Cr³⁺ absorptions (band centered at 554nm) as well as color centers (continuously increasing absorption in the blue region of the spectrum).}		Fig. 35 Polarized UV-VIS-NIR absorption spectra of a violet-blue sapphire (color-changing). The spectra were recorded parallel (blue) and perpendicular to the c-axis (black). The cause of color can interpreted in terms of concentrations of iron (subordinate isolated Fe³⁺, line at 388nm and Fe³⁺ -pairs, line at 450nm), titanium (Fe²⁺-Ti⁴⁺ charge transfer, centered at 565nm and further to the red region of the spectrum) and chromium (isolated Cr³⁺, bands centered at 405-410 and 554nm).

For the large group of E(IM1) enhanced sapphires tested it was found that chemical concentrations (Fe and Cr) were much lower than those found in the natural counterparts of the same color family. The presence of iron (Fe³⁺, Fe³⁺-pairs) cause absorptions in the gemstones typical for yellow sapphires. Chromium (Cr³⁺) and iron (Fe³⁺, Fe³⁺-pairs) were found to contribute to the color of orangy-pink sapphires. However, the iron caused absorptions as determined by UV-VIS-NIR absorption spectroscopy are too weak to explain the extent of the orange color contribution (See Fig. 26 and 27). Two different reasons can be given to explain this: - The presence of other origins of color are present such as color centers related to light elements (Mg, Be)(Lit. 04) The origin of color must be concentrated in certain levels within the gemstones, not necessarily accessible by ED-XRF analysis. This is confirmed by the microscopic studies. ED-XRF cannot see these layers within the gemstones due to the restricted analytical sampling volume. Therefore, the color, as seen face up, cannot be explained sufficiently by the chemical analysis as determined by ED-XRF on the surface. Similar trends, however, such as the correlation of the color with the dominant trace element present, were still possible. The correlation of the chemistry with UV-VIS-NIR spectroscopy is restricted, because UV-VIS-NIR is a bulk analysis which measures the light absorbed in certain colored levels of the gemstone. The intensity of the absorption spectrum depends on the light that passes through the gemstone. The layers of color measured by UV-VIS-NIR spectroscopy may not be at the same position as the part of the gemstone measured by ED-XRF analysis. Therefore, further conclusions are difficult for samples with color zoning. To further clarify the origin of color, it is therefore necessary to extend the ED-XRF to another method which focuses precisely on the micro-chemical variations within the gemstone, and which allows correlation of micro-banding of chemical compositions with microscopic color banding. Further tests require a complicated sampling procedure (See Table 6) and extensive measurements by LA-ICP-MS as shown in the following pages.