Glass filling of highly included natural ruby to significantly enhance clarity

     Once upon a time, a by-product of the routine heat treatment of rubies was the filling of common small surface inclusions with residual flux from the crucibles used in the treatment. Now we are seeing cases where highly imperfect rubies with significant surface fissures have been filled with lead-containing glass that transforms their appearance completely.

     The material used is often very low grade pink, red or purplish Madagascar corundum but glass-filled stones from Tanzania and Myanmar have also been noted. The effectiveness of the treatment is amazing in that it transforms opaque and nearly worthless corundum into material that is transparent enough for use in jewelry. However, ammonia, bleach, and even concentrated lemon juice were found to damage the filler by turning it white at the surface.

     Cavities filled with high-lead glass can be challenging to see under the microscope, but most samples recently examined by the Gemological Institute of America contained gas bubbles and exhibited a blue or orange flash effect at the interface between the natural material and the filler. Glass is significantly softer than ruby, and the filled area may exhibit an inferior polish to that of the natural ruby host.


This month's sources include The Loupe and GIA World News Vol. 15 #3