Lab Notes Gems & Gemology, Summer 2015, Vol. 51, No. 2

Beryllium-Diffused and Lead Glass–Filled Orange Sapphire

Orange sapphire submitted for identification
Figure 1. This 4.25 ct orange sapphire was submitted for a colored stone identification report. Photo by Robison McMurtry.
The Carlsbad laboratory recently received a 4.25 ct transparent orange oval mixed-cut stone (figure 1) for a colored stone identification report. Standard gemological testing established the following properties: RI—1.762 to 1.770; birefringence—0.008; optic sign—uniaxial negative; pleochroism—strong orange and very light yellow; and SG—3.98. All of these properties are consistent with both natural and synthetic sapphire.

Under magnification, the most distinctive internal characteristic was the presence of numerous fractures partially healed by glassy flux-like droplets. Some had a wispy veil and fingerprint-like appearance. One fracture with large healed areas showed a strong flash effect, which appeared greenish blue under brightfield illumination (figure 2, left) and orange-red under darkfield illumination (figure 2, center) when rotating the stone to another direction. Under diffused lighting, the outline of the large partially healed areas stood out clearly (figure 2, right). The flash effect indicates that the fracture was healed using a material with a high refractive index, possibly lead glass (S. F. McClure et al., “Identification and durability of lead glass–filled rubies,” Spring 2006 G&G, pp. 22–34).

Orange sapphire under brightfield, darkfield and diffused lighting
Figure 2. Under brightfield illumination, the large healed fracture showed a greenish blue flash effect (left). Under darkfield illumination, the same fracture showed an orange-red flash effect (center). Under diffused lighting (right), the outline of the fracture became distinct. Photos by Ziyin Sun; field of view 3.45 mm.
Heat-treated sapphires submitted to GIA are routinely checked using advanced analytical tools. Energy-dispersive X-ray fluorescence (EDXRF) analysis was performed on the table facet where the large healed fracture with flash effect was located. The presence of Pb was clearly detected, indicating the filler was lead glass. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) was used to confirm the stone’s natural trace-element chemistry and detect the beryllium-diffusion treatment. Beryllium was present in all laser ablation spots, but analysis of the partially healed fractures revealed high Pb content in addition to Be. In one spot, both high Be (156 ppmw) and Pb (23,300 ppmw) were present, which provided some possible insight into the treatment process.

Corundum selected for lead-glass filling usually goes through a two-step process. The material is heated at temperatures from 900° to 1400°C to remove the impurities in fissures, followed by another heating at 900° to 1000°C in the presence of a lead-glass mixture to fill the fractures (V. Pardieu, “Lead glass filled/repaired rubies,” Asian Institute of Gemological Sciences Gem Testing Laboratory, 2005, Beryllium diffusion requires a temperature range from 1800° to 1850°C, close to corundum’s melting temperature of 2045°C, which requires a special resistance furnace (J. L. Emmett et al., “Beryllium diffusion of ruby and sapphire,” Summer 2003 G&G, pp. 84–135). If the stone had been filled first, the lead-glass filler likely would have melted and flowed out of the fractures during the beryllium diffusion treatment (McClure et al., 2006). It is unlikely that the Be diffusion and lead glass–filling treatments occurred simultaneously. Such a process has not been documented and would likely pose technical challenges, partly because the relatively high temperature required for the beryllium diffusion treatment would have outgassed much of the lead and caused serious toxic gas exposure.

The best possible explanation is that the stone was beryllium diffused at a high temperature to improve its color before being treated with lead glass at a lower temperature to improve its clarity. If some residue of the original Be treatment was left in a fracture that was later filled with lead glass, this could explain the correlation of high Be and high Pb in one of our LA-ICP-MS analyses.

GIA’s laboratories have identified numerous beryllium-diffused or lead glass–filled stones. That the two treatments were applied in a single sapphire is very rare. It is the first time such a stone has been examined by the GIA laboratory.

Ziyin Sun is a staff gemologist in the gem identification department at GIA in Carlsbad, California.