Lead-glass filling is a fracture-filling treatment, applied principally to ruby, in which the surface-reaching fissures of heavily fissured rough are infilled with a molten high-lead-content glass to mask the fissures and dramatically improve the apparent clarity of the finished stone. The treatment was developed around 2003-2005 and entered large-scale commercial use in 2005-2006, mostly applied to low-grade Mozambican, Madagascan and Andilamena rough that would otherwise have had little or no commercial value as faceted stone.

The chemistry is critical. The infill is a high-refractive-index lead glass, with a lead oxide content typically of 30-50 percent by weight, chosen because lead delivers an RI close to that of corundum (around 1.76) and so makes the filled fissure optically near-invisible. Bismuth-based and other lead-free glasses can produce similar optical results but with a less saturated flash effect; the lead version remains the most aggressive and the most widely encountered.

The process involves cleaning the rough, pre-treating with flux, heating to above 1,000 degrees Celsius to fluidise the glass, and holding while the glass is drawn into the fissure network by capillary action. The cooled stone shows a face-up appearance transformed: where the rough showed a network of opaque white fractures, the finished stone shows a clean ruby colour with only a fine ghost trace of the original fracture pattern. The improvement can be dramatic, taking material from essentially un-cuttable to a saleable face-up appearance.

Identification is straightforward at the bench. A fibre-optic light at a glancing angle picks up the characteristic blue flash effect at the filled fissures (sometimes accompanied by an orange or yellow flash), gas bubbles are trapped in the infill, and concavities at the surface mark where the glass has shrunk during cooling. Major laboratories report lead-glass-filled ruby as a composite stone, and the trade is required to disclose the treatment at every transaction.

The value implications are profound. Untreated heavily fissured ruby of comparable visual appearance does not exist; the treatment is the only way to bring this material to market. As a result, lead-glass-filled ruby trades at a fraction of the price of untreated or merely heat-treated ruby of comparable face-up appearance, often less than 10 percent. The treatment is not stable: ultrasonic cleaning, jewellers' pickle, retipping, laser-welder repairs and ordinary wear all degrade the infill over time, and the stones require careful handling in setting and resetting. None of this is visible to the consumer at point of sale, which is why disclosure obligations are particularly strict.