Blue sapphires

Some treatments are practiced so widely that untreated material is essentially unknown ia the jewelry trade. The heating of pale Fe-containing chalcedony to produce red-brown carnelian is one of these. Another example iavolves turquoise where the treated material is far superior ia color stabiUty. Such treatments have traditionally not been disclosed. Almost all blue sapphire on the market has been heat treated, but it is not possible to distinguish whether it was near-colorless comndum containing Fe and Ti before treatment, or whether it had already been blue and was only treated ia an attempt at marginal improvement. The irradiation of colorless topa2 to produce a blue color more iatense than any occurring naturally is, however, self-evident, and treatments used on diamond are always disclosed. 

As a contradistinction to the relatively simple case of AI2O3 Cr(III) where the color is due to a metal-centred electronic transition, we mention now on one hand the fact that the Cr(III) ion colors many transition-metal oxides brown (e.g. rutile Ti02 or the perovskite SrTi03 [15]), and on the other hand the fact that the color of blue sapphire (AI2O3 Fe, Ti [16]) is not simply due to a metal-centred transition. By way of illustration Fig. 1 shows the diffuse reflection spectrum of SrTiOj and SrTi03 Cr(III) [17], and Fig. 2 the absorption spectrum of Al203 Ti(III) and Al203 Ti(III), Fe(III) [18]. It has been shown that these colors are due to MMCT transitions and cannot simply be described by metal-centred transitions [19],... 

Consider first blue sapphire Al203 Ti(III), Fe(III) (Fig. 2). In the absence of Fe(III) the absorption spectrum is easy to interpret. The weak band with a maximum at about 500 nm is due to the t2 —> e crystal-field transition on Ti(III) (3d ), the strong band at 2<280nm is due to a Ti(III)-0( — II) LMCT transition. The absorption band in the region around 700 nm in the case of the codoped crystal cannot be due to Fe(III). It has been ascribed to MMCT, i.e. to a transition within an iron-titanium pair ... 

The importance of MMCT transitions for minerals has been clearly indicated by Burns et al. [68]. After blue sapphire (see above) another famous case... 

The Sephiroth of the Tree change into the Queen Scale of Briah and, simultaneously, you see that the field of the phoenix shield that hangs over the door to the chapel, is changed from white to azure. And within the chapel, the royal orb, cushioned upon the altar, is now formed of precious blue sapphire. 

There are only seven different types of crystal systems, or geometric arrangements, but there are many types of crystals. What makes the difference are the different atoms in each crystal. Blue sapphires and red rubies lAH ... 

Spinel is a colorless magnesium aluminate (MgAl204) of cubic structure. It is hard and durable, but, like white sapphire, it is not a good diamond substitute because it has a low refractive index and lacks brilliance. However, it is readily doped to produce other gems of various colors. Artificial ruby, for example, is often natural red spinel, and most synthetic blue sapphires on the market are actually blue spinel. 

Figure 4.16 Polarized absorption spectra of natural and synthetic sapphires (from Bums and Bums, 1984a). (a) Natural yellow sapphire (b) natural dark blue sapphire (c) synthetic Ti-doped A1203 (d) synthetic Fe-Ti-doped A1203. —Ellc spectra -----EJx...

Representative optical spectra of various sapphires are illustrated in fig. 4.16. It is apparent from the spectra of the natural blue sapphire (fig. 4.16b) that absorption minima in the violet-indigo and blue-green regions, which are located between sharp peaks at 25,680 cm-1 and 22,220 cm-1 and broad bands spanning 17,800 to 14,200 cm-1, are responsible for the blue coloration. Absorption at 17,800 to 14,200 cm-1 is less intense in spectra of natural yellow sapphire (fig. 4.16a see also fig. 3.21) containing negligible Ti. The spectra of synthetic Ti3+-doped A1203 (fig. 4.16c) show absorption maxima at... 

The absorption bands at 18,450 cm-1 and 20,300 cm-1 (fig. 4.16c) represent crystal field transitions within Ti3+ ions, and the weaker band near 12,500 cm-1 may represent a Ti3+ - Ti4+ IVCT transition between cations in face-shared octahedra. The peaks in the spectra of the yellow and blue sapphires clustered at 22,200 cm-1 and near 26,000 cm-1 represent spin-forbidden 6A, - 4AxfE G) and 6A[ — 4A2,4E(D) transitions in octahedrally coordinated Fe3+ ions (fig. 3.10), intensified by exchange interactions between adjacent Fe3+ ion pairs in the corundum structure ( 3.7.3). Other spin-forbidden Fe3+ bands occur at... 

Ferguson, J. Fielding. P. E. (1971) The origins of the colours of yellow, green and blue sapphires. Chem. Phys. Lett., 10,262-5. 

Townsend, M. G. (1968) Visible charge transfer band in blue sapphire. Solid State Comm., 6, 81-3. 

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