LMHC Labs Agree on Glass-Filled Ruby

Prior to treatment, much of the low-quality, highly fractured ruby that comes from Madagascar is unusable. However, masking these fractures by introducing a glass filler during treatment with low heat results in a relatively transparent commercial gem product, which raises the question of value.

Meanwhile, the seven member laboratories of the Laboratory Manual Harmonization Committee are addressing the issue of how best to describe the filler. LMHC members are the American Gem Trade Association Gemological Testing Center (United States), CISGEM (Milan, Italy), GAAJ (Japan), GIA (United States), GIT (Thailand), Gübelin Gem Lab (Switzerland), and SSEF (Switzerland).

Many of the LMHC labs have agreed on standardized nomenclature to describe glass-filled fissures and cavities when there are indications that the clarity of the corundum has been enhanced or modified by the low-heat introduction of the glass.

Reports may now read:

Species: natural corundum;

Variety: ruby or sapphire;


  1. (Indications of) Clarity enhancement/modification by a glass filler in fissures;

  2. Glass-filled fissures;

  3. Glass in fissures.

Reports will also include the appropriate filler quantification terminology (alphanumeric and/or text description), the identification of the glass material (e.g., lead, glass, silica glass, or other material), and/or one of the following statements:

  1. No indications of clarity enhancement, or clarity modification, or glass in fissures;

  2. F1 – Minor clarity enhancement, or clarity modification by a glass filler in fissures;

  3. F2 – Moderate clarity enhancement, or clarity modification by a glass filler in fissures;

  4. F3 – Significant clarity enhancement, or clarity modification by a glass filler in fissures.

Further optional report comments include:

  1. A lead glass, or a silica glass, etc., has been identified as the filler;

  2. The introduction of glass into fissures involves heating.

The glass filler in question is not what most retailer jewelers are familiar with. “Traditional high-temperature heat treatment” helps dissolve opposing fracture walls, which can partially reconnect upon cooling. Much of the fracture walls actually grow back together. This is called partial healing, and the residue left inside the partially healed fracture is a glass filling. This is a permanent process, and not the glass filler used in low-temperature masking.