Lead Glass–Filled Black Star Sapphire
Fracture and cavity filling of gemstones has been practiced for hundreds of years. In recent times, this treatment has become more sophisticated and widespread, particularly with the use of lead glass, which is most often seen in rubies. The addition of lead oxide (PbO) to silica glass lowers its working temperature and viscosity while raising its refractive index from approximately 1.5 to as high as 1.8. Thus, the filler has an RI that matches many gem materials, making fractures and voids virtually disappear. But the softness and low melting point of the filler create durability issues, especially in low-quality filled rubies (S.F. McClure et al., “Identification and durability of lead glass–filled rubies,” Spring 2006 G&G, pp. 22–34).
Common inclusion features for lead glass–filled ruby composites include blue flashes from the fractures and trapped gas bubbles. Glass features can also be detected using Fourier-transform infrared (FTIR) spectrometry and energy-dispersive X-ray fluorescence (EDXRF) spectroscopy. But if the ruby has very few areas that are enhanced with lead glass, they may not be obvious under magnification, and FTIR and EDXRF may not detect them. Real-time X-ray (RTX) imaging systems, used by GIA to identify pearl growth features and confirm filling in diamonds, can be beneficial in differentiating between lead glass and ordinary glass filling in rubies and other gemstones. This is due to opacity differences, as lead glass has a lighter appearance than glass without lead. A combination of X-ray imaging and magnification is also capable of quantifying the extent of lead glass treatment performed on a gemstone.
A 25.10 ct opaque black star sapphire (figure 1) was submitted to the New York laboratory for identification. The stone contained a few hairline fractures and dense golden silk needles. At first glance, the sapphire had a few features that would arouse suspicion for treatment, such as round cavities on the base and a few inconspicuous fractures (figure 2). The sapphire also had a specific gravity of 4.04, which is above the normal range for corundum. Natural black sapphire may have a higher SG due to abundant asterism-causing inclusions of rutile or hematite, both of which are denser than corundum. Because of the stone’s opacity, it was difficult to observe internal inclusions under magnification.
Using X-ray imaging analysis (figure 3), it was clear that the fractures and multiple cavities had been filled with lead glass, and this finding was confirmed by EDXRF. These properties would have been extremely difficult to see under normal visual magnification.
This is the first example of a lead glass–filled black star sapphire examined in the New York laboratory.
