Lead-free glass filling is a fracture-filling treatment for fissured corundum (chiefly ruby) in which the cavities and surface-reaching feathers of the rough are infilled with a molten high-refractive-index glass that does not contain lead. It is positioned in the trade as a less politically and toxicologically problematic alternative to the dominant lead-glass filling that has been used on heavily fissured ruby since the early 2000s. The chemistry generally relies on bismuth-bearing glasses, occasionally on other heavy-element compositions, all chosen to deliver a refractive index close to that of corundum (around 1.76) so that the infill remains optically inconspicuous.

The treatment process closely mirrors that of lead-glass filling. Heavily fissured rough is cleaned, pre-treated with flux, heated in a furnace under controlled atmosphere to a temperature at which the glass becomes fluid (somewhat above 1,000 degrees Celsius, depending on the glass composition), and held while the glass is drawn into the open fissures by capillary action. Cooling proceeds slowly to avoid thermal shock. The finished stone shows a face-up appearance dramatically improved over the untreated rough, with much of the fracture network now filled and optically continuous.

From an identification standpoint, lead-free fillings are detectable by the same diagnostic features as lead-glass fillings: residual gas bubbles trapped in the infill, blue or orange "flash effect" under fibre-optic illumination at the filled fracture, and characteristic surface concavities where filler has shrunk during cooling. SEM-EDS analysis at major laboratories identifies the bismuth or other heavy-metal signature and distinguishes the filling from lead-glass at composition level.

The trade significance of lead-free filling is twofold. It addresses the regulatory concern around leaded products in jurisdictions where lead content is restricted at retail (notably for jewellery sold to children in the United States and the European Union). It does not address the underlying durability concern, however: lead-free glass fillings remain materially less stable than untreated ruby, can be damaged by ultrasonic cleaning, by exposure to acids encountered in jewellers' pickle and rhodium-plating processes, and by the heat of routine bench repair work. Disclosure obligations are identical to those for lead-glass filling: full disclosure to the buyer at every transaction, with a clear note that the stone has been composite-treated.