corundum, properties.3, synthetic corundum.7, inclusions.5, gems

It has been generally accepted that asterism in corundum (star rubies and sapphires) is caused by needles of rutile (TiO₂) aligned along crystallographic planes of the corundum (e.g. Webster, 1962). The evidence given by Tait (1955) was: (1) the presence of titanium, (2) the needle-like habit, and (3) the square cross-section of the needles. Based on the analogy with rutilated quartz, and by eliminating octahedrite and brookite by the morphology, rutile was accordingly identified. This type of evidence, however, is explained equally well by either TiO₂ or the compound aluminum titanate Al₂TiO₆.