Since the mid-2000s the market has been supplied with large quantities of transparent red stones, of impressive size and low price, that test as corundum on the refractometer. Many are glass-filled ruby: low-grade, heavily fractured material that has been impregnated with high-refractive-index lead glass until the fractures disappear and the stone looks clean. The process is so extensive in some stones that laboratories classify them not as treated ruby but as a composite of ruby and glass.
How it is done
Heavily fractured, opaque or near-opaque corundum — material that would otherwise be cut into beads or discarded — is cleaned, sometimes with acid, and heated together with a lead-rich glass powder. The glass melts and is drawn into every fracture by capillary action. On cooling, the fractures are filled with a material whose refractive index is close to that of corundum, so light passes through instead of scattering, and the stone becomes transparent.
The volume of glass can be considerable. In heavily fractured material a substantial fraction of the finished stone by volume may be glass.
Why it matters
Value. The starting material is nearly worthless. The finished stone looks like a ruby worth many times more. Sold with full disclosure at a price reflecting the treatment, it is an inexpensive red stone; sold as "natural ruby", it is a fraud of large proportion.
Durability. Lead glass is soft, has a low melting point and is attacked by acids. A glass-filled ruby can be damaged by a jeweller's torch during setting or repair, by ultrasonic and steam cleaning, by household chemicals including lemon juice, and by ordinary wear. The glass can etch, craze, or leave the fractures, and the stone returns to its fractured state.
Identity. At high glass content the stone is no longer simply a treated ruby. It is a manufactured composite, and a report should say so.
What the laboratory sees
Gas bubbles in fractures. Round or flattened bubbles trapped in the glass as it filled the fractures. The most reliable single sign.
Flash effect. Where a filled fracture is viewed at a glancing angle, the small difference in RI between glass and corundum produces a coloured flash — typically blue, sometimes orange — that moves as the stone is tilted. Unfilled fractures do not flash.
Crazed or crackled texture. The glass within larger fractures often shows a fine network of cracks from cooling, visible under magnification.
Surface features. Glass at the surface polishes differently from corundum and may show as lower-lustre patches, pits or a slightly different texture where a fracture meets a facet.
Colour of the glass. Sometimes the glass is itself coloured to enhance the red, and the colour is concentrated in the fractures.
Reaction to acid or heat — not used by the laboratory, because the tests are destructive, but the reason for the care warning on the report.
Glass filling is not subtle under a microscope. It is missed only when no one looks.
Related treatments
Older, lighter fracture-healing with flux or borax during heating leaves glassy residues in fissures that are also reported, but the volume is small and the stone is fundamentally a heated ruby. The line between "heated with residues" and "glass-filled composite" is drawn on the extent of the filling, and laboratories describe the degree — minor, moderate, significant.
What a report should say
Whether filling is present, its extent, and the nature of the filler where determinable. A conclusion such as natural corundum (ruby), glass-filled — composite material tells a buyer exactly what they have. A report that says only natural ruby on a glass-filled stone has failed at the point that matters.
Frequently asked questions
Is glass-filled ruby real ruby?
The corundum is natural. The stone as sold is a composite of that corundum and manufactured glass. Whether that counts as "real ruby" depends on what the buyer was told and paid.
Can glass filling be removed?
Not without returning the stone to its fractured, opaque condition. The filling is what makes it look like a gem.
How can I tell if my ruby is glass-filled?
Under a loupe, look into any visible fractures for bubbles and for a blue flash as you tilt the stone. A laboratory examination is definitive and inexpensive relative to the value question.
Is it safe to wear?
With care — no ultrasonic or steam cleaning, no chemicals, no heat from repairs — it can be worn. It is far less durable than untreated or heated ruby, and the report's care note should be followed.
Does glass filling affect the refractive index?
Corundum reads 1.762–1.770; the glass is chosen to be close. A facet crossing a large filled fracture can give an anomalous reading, which is a clue, but the RI alone does not detect filling. The microscope does.