Green kyanites are quite rare, the yellow ones are even rarer, the 7.69 ct sample presented here is reportedly from Madagascar but because of its Mn content it might be coming from Tanzania as for the orange kyanite from Loliondo. Such a big faceted yellow kyanite is quite unusual.
The material is not clean as it can be observed on figure 1. The gemstone is pretty well polished considering how difficult it is to factet kyanite. The material shows several fractures but it is nevertheless transparent. Some small orange 'inclusions' are present. It has not been investigated yet if they are another material or just color variations close to the original crystal's faces.
Figure 1. Rare 7.69 ct yellow kyanite reportedly from Madagascar,but more likely from Tanzania.
| Shape | 'emerald cut' |
| Size |
16.2 x 8.8 x 5.1 mm |
| Color | yellow |
| Lustre | vitreous |
| Weight | 7.69 ct |
| SG | 3.68 |
| RI | 1.717 - 1.734 |
| DR | 0.017 B+ |
| Pleochroism |
orangish-yellow / light-yellow |
| Polariscope / Conoscope | light/dark 4 times / 360° |
| SWUV | inert |
| LWUV | inert |
| Magnetic susceptibility | weak with N52 |
Table 1. Observational and measured properties
Infrared reflectance spectroscopy:
The IR reflectance spectrum (figure 2), acquired from the table, exhibits the characteristic spectrum of kyanite specie.
Figure 2. IR reflectance spectrum of the 7.69 ct yellow kyanite, acquired from the table, exibits the characteristic spectrum of kyanite specie. UV-VIS-NIR spectroscopy:
The UV-Vis-NIR absorption spectrum (figure 3) was acquired with the light path perpendicular to the table with the light entering the pavilion.
The unpolarized UV-Vis-NIR absorption spectrum exhibits features at 436, 447, 468, and 490 nm, all combined producing a strong band centered approximately at 450 nm. The rising edge towards UV (410 - 390 nm) suggests the 370-380 nm bands usually observed in Fe3+-kyanite. There is a weak feature at 555 nm followed by a weak and large band around 620 nm in the NIR direction of the spectrum. Two very comon weak bands are also present at 691 and 710 nm as observed in most of kyanites. NIR region shows a moderately strong and very wide band (FWHM > 200 nm) centered around 1000 nm. The 380, 436, 447 and 620 nm features are related to Fe3+ as for green kyanites (Faye and Nickel, 1969)[2], (Parkin et al., 1977)[1]. The 691 and 710 nm are unambiguously attributed to Cr3+. The rare features at 468, 490, 555 and ~1000 nm are possibly related to Mn3+ as for orange kyanite mined in Loliondo region in Tanzania.
The extra absorption in the 460 - 550 nm region that does not exist in the green kyanite spectra explains the yellow color.
Figure 3. Unpolarized UV-Vis-NIR absorption spectrum of the 7.69 ct yellow kyanite exhibiting features at (380,) 436, 447 and 620 nm related to Fe3+, at 468, 490, 555 and 1000 nm attributed to Mn2+, and at 691 and 710 nm the common Cr3+ features.Photoluminescence spectroscopy:
Although the gemstone is inert under the SW and LW lamps, a 405 nm laser pointer induce red luminescence. The photoluminescence spectrum (figure 4) excited with a 405 nm laser was acquired without any specific orientation.
Figure 4. Photoluminescence spectrum of the 7.69 ct yellow kyanite showing the Cr3+ R2 & R1 lines respectively at 689 and 707 nm as well as the 760 nm band attributed to another Cr3+ emission center. Unlike most of kyanites, the spectrum shows another emission center in the 575 nm region, it is possibly attributed to Mn2+.This yellow kyanite shows a moderate red luminescence attributed to the R2 & R1 lines of Cr3+ respectively at 689 and 707 nm (Gaft et al.)[5], the R lines being associated to the Cr3+ in site A. The 760 nm emission center (Cr3+ in site B) is also present although it does not clearly appear in the spectrum of figure 4. Unlike most of kyanites, the spectrum shows another emission center in the 575 nm region, rather weak compared to Cr3+ emission centers. Such emission center is consistent with Mn3+ emission centers observed in many minerals, as well as it is consistent with the Mn3+ absorptions features (excitation features) observed in the UV-Vis-NIR spectrum of figure 3.
Conclusion:
The yellow color of this unusual 7.69 ct yellow kyanite is due to Fe3+ and Mn3+ ions.
[1] Mineralogical Applications of Crystal Field Theory, R. G. Burns, 2005, ISBN: 0521017858 | 978-0521017855
[2] On the origin of colour and pleohroism of kyanite, G. H. Faye, E. H. Nickel, The Canadian Mineralogist, Vol 30, No 35, 1969
[3] Blue colour-changing kyanite from East Africa, G. Bosshart et al., Journal of Gemmology, Vol. 18, No. 3, pp. 205–212
[4] Kyanite Visible Spectra - List of Visible Data Files on the Caltech Mineral Spectroscopy Server
[5] Modern Luminescence Spectroscopy of Minerals and Materials, 2nd Edition, M. Gaft, R. Reisfeld, G. Panczer, Springer Editor, ISBN: 9783319247632