Lead-glass residue is the visible deposit of residual lead-glass infill at the surface of a composite-treated ruby, observed either as small pools, thin films or bubble-bearing patches that reach the polished facets. It is one of the diagnostic surface features of lead-glass filling, alongside the more widely-cited blue flash effect along treated fissures, and is observable at 10x with a loupe under fibre-optic illumination.

The residue arises because the molten glass, drawn into the fissure network of the rough by capillary action during the treatment process, sometimes pools at fissure mouths where they break out at the surface or fills small concavities adjacent to those mouths. After cooling and final polishing, those residue zones may be partly polished away but generally leave traces: a slight surface concavity, a glassy patch of slightly lower hardness than the surrounding corundum, and trapped gas bubbles within the patch. Under high magnification the boundary between the corundum and the glass is visible as a thin compositional discontinuity.

The diagnostic importance of lead-glass residue is that it appears at the surface, accessible to non-destructive bench inspection, and so confirms composite treatment without need for laboratory composition analysis. It is, in effect, one of the cheapest and most reliable bench-level identification clues available for the trade, and its presence on a stone marketed as untreated ruby is grounds for either a hard pass or a renegotiation of price down to composite-treated levels.