corundum, treatments.6, dye, heat, gems

A small (4 mm in diameter) natural corundum bead recently examined in the GIA Applied Gemology Department probably owes most of its purplish pink bodycolor to the red substance that was found to be lining the drill hole (figure 10). In the past, we have frequently seen near-colorless corundum beads take on a pinkish cast when they were strung on a red cord, but this is one of the few times we have encountered a color-coated drill hole. Of course, it is impossible to judge the true colors of such items without removing the coating in the drill hole. Heat treatment is commonly known to produce blue and shades of yellow to orange-brown sapphires from milky lighter colored sapphires and to enhance rubies by minimizing their blue component. Recently, however, the New York lab encountered heat-treated stones in colors they have not seen previously. The strain discs evident in the beautiful 4-ct red-purple sapphire shown in figure 13 suggest that the stone owes its color, at least in part, to heat treatment. But the zones of chalky fluorescence that appeared when the stone was exposed to short-wave ultraviolet radiation are proof positive that the stone has been heat treated.