Linde stars

PRODUCTION OF ASTERIATED SYNTHETIC RUBY AND SAPPHIRE
In synthetic ruby and sapphire, asterism is produced in a two-step process:

Growth of a non-asteriated crystal that contains distinct amounts of titanium in the corundum lattice
Heat treatment of the homogeneous titanium-bearing sample to form needle-like precipitates of a titanium-bearing phase
Two variants are known for the first step of this process. In one variant, titanium oxide is added to the aluminum oxide powder used for the production of synthetic ruby and sapphire boules by the Verneuil method (Burdick and Glenn, 1949; Burdick and Jones, 1954a) or by the Czochralski technique (Keig et al., 1972). In another variant, titanium is diffused into whole and half boules as well as pre-cut cabochons of synthetic Verneuil-grown corundum (Eversole and Burdick, 1954c; see also Carr and Nisevich, 1975a, 1976a, 1977).

After either variant of the first production step, heat treatment is performed for the formation of needle-like precipitates in the corundum matrix. This step exsolves three differently oriented series of needles, composed either of rutile or of another titanium-bearing phase. Reflection and scattering of light from these needles are then responsible for the three intersecting light bands of asteriated rubies and sapphires. The exsolution step is necessary regardless of which variant of the first production step is employed.

Numerous papers have dealt with the examination and identification of the needle-like precipitates formed in Ti-doped corundum after annealing at temperatures in the range of 1500° to 1100°C (see references below and patent documents in table A-1). The different phases present as acicular inclusions have been characterized in the transmission electron microscope using electron diffraction and/or other microanalytical techniques. It has often been noted that twinned rutile needles are formed, but the exsolution of rhombic or monoclinic TiO2 phases and even the formation of Al2TiO5 have been reported as well (Phillips et al., 1980; Langensiepen et al., 1983; Moon and Phillips, 1991; Xiao et al., 1997; Viti and Ferrari, 2006; He et al., 2011). For simplicity, we shall only mention “rutile” needles or precipitates, aware of the fact that other Ti-bearing phases might also be present.

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