It is always Questionable if the Glass filled ruby is even Natural. Well It is !! According to the GIA, “the GAAJ laboratory in Japan issued a lab alert about rubies they had seen that had large numbers of fractures filled with high-lead-content glass, which made them appear very transparent. Since then, large quantities of this material have reached international markets.”
The material(raw) is of very low quality: opaque, industrial-grade stones, rather brownish than red. Until lead glass-filling was introduced, such material could be used as mineral specimens only. It could not be used even for cheap fashion or costume jewelry.
This low-quality corundum, that has an enormous amount of fractures and cavities, is mixed with powders that are composed primarily of lead and silica but may also contain other ingredients, such as copper or bismuth. This mixture is heated fusing the powders into a lead glass that penetrates the fractures and cavities in the stone.
According to GIA, “The effectiveness of the treatment is amazing, in that it transforms corundum that is opaque and nearly worthless into material that is transparent enough for use in jewelry.”