synthetic corundum.7, gems, inclusions

This digest reports repeated twinning lines in synthetic corundum examined at the Gem Trade Laboratory of the Gemological Institute of America, challenging the assumption that such features establish natural origin. The principal example is a transparent blue synthetic sapphire discovered during the identification of a group of blue stones. It displayed both curved striae, regarded as evidence of synthetic origin, and repeated twinning lines, previously considered exclusive to natural material. The accompanying photograph records the coexistence of these features in the same specimen. An earlier laboratory observation involved a yellow synthetic sapphire with a single line described as a repeated twinning line. Subsequent examination of both the yellow and blue sapphires in a polariscope revealed irregular extinction resembling that of naturally occurring repeatedly twinned corundum. Thus, neither the visible twinning features nor the associated extinction behavior provided an exclusive distinction between natural and synthetic stones. A further example appeared only a few days later, when a dealer submitted a 5.6-carat cabochon ruby showing prominent repeated twinning lines. The dealer had purchased it as natural specifically because of these features, but laboratory examination identified it as synthetic. Its dark color prevented an adequate photograph for publication. Together, the three observations demonstrate the practical danger of accepting a familiar structural feature as sufficient evidence of natural origin without continuing the search for indications of synthesis. The blue sapphire is particularly instructive because its curved striae and twinning lines bring two previously contrasting identification criteria together within one stone. The ruby illustrates the commercial consequence of relying on twinning alone: a synthetic specimen had already been purchased under an incorrect identification. The digest therefore advises further examination for synthetic features whenever repeated twinning is observed, rather than treating its presence as conclusive. It also draws an educational distinction between the curved pattern of striae and the crystallographic organization of synthetic corundum, cautioning against the beginner’s assumption that crystallization itself follows those curves. The report presents these cases as a correction to identification practice, extending the recognized range of structural appearances in synthetic ruby and sapphire.