The asterism in star sapphire is due to precipitation of needle-like particles in the basal plane of sapphire. These particles have now been identified as rutile (TiO2) by both electron diffraction and microchemical analysis. The rutile needles have the following orientation relationship with the host sapphire: 〈011〉R‖〈1010〉S {011}R‖{1110}S where the first relation defines the needle axis. This orientation occurs because of the good lattice matching it provides.
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Phillips et al. (1980) studied this question.
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