A lead-glass-filled sapphire is a sapphire (blue, fancy-colour or near-colourless) in which the surface-reaching fissures have been infilled with a molten high-lead-content glass to mask the fissures and improve the apparent clarity of the stone. The treatment is the same in principle as the much more widespread lead-glass filling of ruby, but it has historically been applied less aggressively to sapphire because heavily fissured sapphire rough is less common in the supply chain than equivalent ruby, and because the colour-disguise economics that drive the ruby treatment are weaker for blue sapphire.

Identification is by the same suite of microscopic clues as for lead-glass-filled ruby: residual gas bubbles trapped in the infill, characteristic blue-violet flash effect under fibre-optic illumination at the filled fracture, and surface depressions where filler has shrunk during cooling. Where the sapphire is light blue or near-colourless, the flash effect is sometimes more difficult to see against the body colour than it is in ruby. SEM-EDS at major laboratories provides definitive identification of the lead-glass composition.

From a trade and value standpoint, lead-glass filling carries a steep value penalty. The treatment is not stable: the infill softens at temperatures encountered in routine bench work (laser welding adjacent to the stone, retipping, even prolonged exposure to jewellers' pickle), can be damaged or removed by ultrasonic cleaning, and degrades over time with normal wear. Disclosure obligations are accordingly strict: all major laboratories report lead-glass-filled sapphire as a composite stone, and AGTA and equivalent guidelines require full disclosure at every transaction in the trade. Value is typically a fraction of an equivalent untreated stone, often 10 percent or less, and the stone is sold by appearance rather than by carat weight in the same sense as natural sapphire.