corundum, color.4, gems

It is widely accepted that the blue color in sapphire is due to Fe²⁺/Ti⁴⁺ charge transfer, often accompanied by Fe²⁺/Fe³⁺ intervalence charge transfer (IVCT; Schmetzer and Bank, 1981; Fritsch and Rossman, 1988). However, to date the extent of the contribution of the Fe²⁺/Fe³⁺ IVCT to the color of sapphire has not been established, since no sapphire has been reported in which this chromogen contributes the primary absorption. We are now writing to report that such material has recently been studied, and it proves that Fe²⁺/Fe³⁺ IVCT alone can produce a blue color in sapphire. The two grayish blue sapphires examined are 1.57 ct and 1.19 ct (figure A). Their UV–visible–near infrared spectra (figure B) reveal a broad band centered at 890 nm, which corresponds to the broad band centered at 870 nm that has been attributed by Ferguson and Fielding (1971) and Schmetzer and Bank (1981) to Fe²⁺/Fe³⁺ IVCT. The other distinct features—at 376, 388, and 451 nm—are due to the Fe³⁺ ion or to Fe³⁺ pair transitions, according to the same researchers. The spectra shown in figure B exhibit little or no absorption due to Fe²⁺/Ti⁴⁺ (which typically occurs at approximately 560 nm), although the stones appear distinctly grayish blue. To our knowledge, this is the first time that Fe²⁺/Fe³⁺ intervalence charge transfer has been demonstrated to cause blue color in sapphire. Note that the blue color generated by this mechanism appears to be significantly grayer than the color produced by Fe²⁺/Ti⁴⁺ charge transfer. Further investigations are being conducted into the polarized absorption spectra of these samples and the response of the Fe²⁺/Fe³⁺ IVCT absorption to various treatment processes. Fe²⁺/Fe³⁺ IVCT plays a major role in the color of many dark blue sapphires from basaltic origins, such as localities in Australia, China, Thailand, and Cambodia (Schmetzer and Kiefert, 1990; Poirot, 1992). Many such sapphires would benefit from having their color lightened, but traditional heat-treatment processes—designed to reduce the Fe²⁺/Ti⁴⁺ charge-transfer absorption in sapphires of metamorphic origin (from localities such as Sri Lanka, Myanmar)—may not be effective in these cases.