Professional Jeweler Archive: Ranking Ruby Treatment

December 2000

Gemstones/Gemology


Ranking Ruby Treatment

Gübelin Gem Lab now classifies the various degrees of thermal enhancement of ruby


The Gübelin Gem Lab, Lucerne, Switzerland, now classifies and discloses on the front page of its gemstone reports the degree to which a ruby has been enhanced.

For centuries, most rubies and sapphires mined throughout the world have been subject to some form of enhancement. Heating, one of the most common forms, dissolves rutile inclusions in the gem and any natural mica present in fissures; it also can trigger the dissolution of the fissure walls, internally healing them and enhancing color and clarity.

Another common treatment involves adding a mineral, most often borax, during the heating process. This triggers similar healing of surface-reaching fissures, fractures or pits.

Gübelin decided to classify ruby enhancement because of "rampant confusion" about what's involved, says the lab's Christian Dunaigre, who conducted a ruby study and helped design the classification system. The system classifies rubies this way:

Here are descriptions of these categories and photographs to illustrate them:

NTE – No signs of thermal (heat) enhancement.


TE1 – No healed fractures are present or only a small number of fractures consisting primarily of tiny droplets and fine tubules.

TE2 – The healed fractures consist primarily of tiny droplets and small tubules.

TE3 – The size and number of healed fractures increases. They consist primarily of tiny droplets, small tubules and fine thin films.

TE4 – The healed fractures are more extensive and consist of small droplets, networks of tubules and thin films.

TE5 – The partially healed fractures consist of coarse droplets, networks of coarse tubules and thick films. Small cavities with residue may be present.

TE6 – In addition to the features described in classifications T1-T5, these rubies have larger cavities filled with residue.

Ruby Study

The Gübelin lab focused on the degree of fissure healing and the amount of residue present. It's difficult to differentiate whether the residue or remnants of a healed fissure occurred because of natural minerals present in the ruby or because of additives.

The lab studied hundreds of rough and faceted rubies – heated and non-heated, with and without chemical additives – from a variety of dealers and sources.

Microscopy, scanning electron microscopy and Raman spectroscopy were used. Microscopy revealed that the appearance of healing fissures with and without chemicals looked virtually the same.

Scanning electron microscopy found a wide variety of chemical compositions in rubies from various suppliers. It also found that residue in fissures as well as cavities of the same ruby were the same compositionally.

Raman spectroscopy showed that substances in the healing fissures and cavities were amorphous. It also found that natural mica present in the fissures goes into solution and triggers the dissolution of fissure walls during heating.

This material is often called glass filling in the trade, but the Gübelin lab says that term is misleading because it leaves the impression the healing of fissures is always intentional and involves coating the gem with borax solutions. The lab stresses it's not always possible to tell whether the healing occurred with natural residue or with an additive.

Gübelin also stresses the classification system is not a description of quality and does not consider the extent of other naturally occurring inclusions. It simply indicates whether a ruby has been heated and classifies the extent of healed fractures and cavity fillings.

The lab says the classification system is easy to use and explain because every ruby report carries the disclosure on the first page and includes a separate graphic and verbal explanation of the categories.

• Gübelin Gem Lab, Lucerne, Switzerland; (41-41) 429-1717, www.Gubelinlab.com.

– Robert Weldon, G.G.



Why Enhance?

Here's what's involved in ruby treatment

Consumers' unrealistic expectations of how a ruby should look fed the demand for sophisticated heating technology, says Christian Dunaigre, who conducted a study of rubies at the Gübelin Gem Lab, Lucerne, Switzerland. The lab's report describes the forms and effects of thermal enhancement of ruby and sapphire and is reprinted below. Ruby and sapphire are forms of corundum.

Color Alteration

The appearance of some varieties of corundum can be altered subtly or dramatically by the generation or removal of certain colors. However, since the gemological labs do not have access to the stones prior to treatment, it is generally not possible to determine the degree to which color of a gemstone has been modified.

Dissolving Rutile Inclusions to Increase Transparency

Dense concentrations of rutile needles, generally referred to as silk, may be reduced or completely removed. Depending on the temperatures and conditions applied during the heating process, a gemological lab may or may not be able to assess the amount of rutile inclusions that were present in these areas of the gemstone prior to treatment.

Healing of Fractures

Corundum commonly comes from the mine with fractures (cracks). This is especially so in the case of rubies. During heat treatment, it is possible to heal, or seal, these fractures, thereby reducing their visibility and improving overall durability of the gems. The effects of this may readily be seen by an experienced gemologist with the use of a microscope or loupe.

Filling Open Fractures and Cavities

During the process of heat treatment, molten substances that assist the healing process may penetrate and fill the open fractures and surface cavities present in some of the gems. These substances then solidify and typically become vitreous. The resulting material, commonly known as a residue, can be easily distinguished from corundum with the aid of a microscope, particularly when the surface of the gemstone is examined in reflected light, owing to the difference in surface luster.

– Robert Weldon, G.G.

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