Growth tubes are a typical inclusion in corundum and often seen during examination in the laboratory. While commonplace, a stunning cluster such as the one seen above calls out to the photomicrographer. At the surface these tubes are cut through and some black debris has become lodged inside the ends, while deeper within the stone they still display a fiery glow, reminiscent of rockets launching into the sky.
Not only do these tubes spark the imagination, but they can actually tell us a lot about the identity of their sapphire host. The limonitic staining found deeper in the tubes is a yellowish orange, suggesting that they have not been altered by heat treatment. These stains often turn a darker reddish or brownish color in heat-treated stones (J.I. Koivula, “Useful visual clue indicating corundum heat treatment,” Fall 2013 G&G, pp. 160–161).
Furthermore, we have observed in the past several months that these prominent growth tubes are a common feature in Madagascar sapphire. While the tubes can be found in corundum from other origins, they seem particularly common in the Madagascar material that has been entering the Bangkok market in greater numbers recently. When taken into account along with other features, they provide a clue to the origin of these sapphires. Thus, what initially appeared to be a mundane inclusion is both visually impressive and gemologically significant, as it provides hints to the origins and treatments of corundum.
The Carlsbad and New York laboratories and schools will be closed on Monday, May 28 in observance of the Memorial Day holiday. These locations will resume regular business hours on Tuesday, May 29.