These terms are used under the comments section of the Report when no indications of heating have been found after analysis of the stone. Note that certain type of treatments, such as low temperature treatments can not always be detected. It may also not be possible to detect the heat treatment used on some type of gemstones such as quartzes, tourmalines, Beryls and others. The fact that no mention of heating or that no indications of heating is stated on the report does not necessarily imply that such a treatment has not taken place.

These generic terms apply for any kind of gemstones when we found indications of a heat treatment process either at low or at high temperatures.
Low temperature is routinely applied to a variety of Corundum to improve their colour appearance (i.e remove or reduce undesirable colours, or enhance some existing ones). Almost all type of fancy colour sapphires can be treated at low temperature to improve their colour appearance. Often referred to as “blow-pipe” these low temperature treatments may also be performed in more regular types of furnaces (i.e LAK gas furnaces) and normally imply temperatures below 1200 degree centigrades.
In all these instances, it can be quite challenging for the gemologist performing the analysis to detect such a treatment as the microscopic features are not always affected. Other analytical techniques may prove, at times, useful in such a separation.

In rubies and sapphires, heat treatment is commonly performed at elevated temperatures, often (but not always) with the addition of chemicals additives, such as Borax, to improve their colour appearance and/or their transparency. These additives will assist in the healing of pre-existing fissures by dissolving the wall of the fissures and eventually become trapped as individual remnants during the healing process. Being enclosed in the corundum matrix, they are therefore permanent and can not be removed by acid cleaning and polishing.

Surface reaching cavities may also be filled by these additives during the heating process and are often referred to in the trade as “Glass filling”. On the contrary to the remnants trapped within healing fissures, these filled surface cavities are not permanent and may be removed by polishing or acid cleaning.

It is the actual amount and state of these remnants of the heat treatment trapped within the fissures that some Laboratories have decided to grade using an alphanumerical or letter type of system (TE1-5, H etc…). In other words, the more of these remnants in fissures are detected and the coarser they are, the lower will be their actual grading. In effect, the increasing presence of these remnants affects the transparency of the gemstone (mostly rubies for that matter). So in most of the cases, gemstones graded TE3-TE5 or Hb-Hc will display a lower transparency than stones with higher grades. Filled cavities are usually graded with a separate nomenclature, but in a different way by various laboratories.

Much effort has been put by some gem testing laboratories to harmonize the reporting nomenclatures between them. These endeavours certainly help the trade by providing a common language when talking about heat treatment. However a lot of confusion still persists as to the meaning and interpretation of the above mentioned systems.

For this reason, C. Dunaigre Consulting has decided to only indicate the presence of heat treatment with the statement Indications of heating under the comment section of the Report, irrespective of the amount of remnants present within the fissures. As mentioned above, the increasing presence and coarseness of the constituents will greatly affect the transparency of the gemstone. Transparency is one of the attributes (together with saturation of colour and cut) which is taken into account to give an optional quality grade that will be stated on our Reports. Corundum with a large amount of remnants within fissures are given lower quality grades as transparency is affected. If filled surface cavities are detected, their presence will be stated as a foot note below the comment section as follows: Indications of filled surface cavities.

More recently, a new development in the heat treatment of corundum surfaced in the market. The process involves the high temperature treatment of sapphires at low pressures (less than 1Kbar) with a modified autoclave originally designed to be used in the high pressure-high temperature (HPHT) treatment of diamonds. This new technique allows for a much faster treatment time (a matter of minutes) whereas it takes several hours or even days with a traditional oven. The result can be quite spectacular and may induce a dramatic change of colour to the stone. As this is the case with traditional heat treatment, this new heating technique will assist in the healing of pre-existing fissures and may also create new tensions within the sapphire.

Disclosure: In accordance with the current findings of a group of Gem testing laboratories which put together resources to investigate claims of stability and durability issues, it was concluded that this new technique does not pose such a threat to the gemstone being treated. We therefore decided to consider it as a simple evolution of the art of heat treatment which did not require a specific disclosure other than the statement Indications of heating under the comment section. Research on the topic is ongoing and we reserve the right to modify that statement if new findings came to change our current stance.

When such a treatment is detected, the following is stated under the identification section of the Report: Lead Glass-treated ruby (treated Corundum). A separate statement detailing the effects of this treatment is also made as a footnote.
When the extent of the treatment is too important, an additional statement that this type of rubies is commonly referred to as “Composite ruby” is also made.

In order to improve /change its color, Corundum may be treated by a diffusion process. The process involves the introduction of a foreign element in the atomic structure of the corundum by heating the stone at high temperature in the presence of different chemical elements.
Titanium was used originally for such a purpose and created a subsurface layer of deep blue colour that would dramatically change the colour of the original corundum. Around the year 2000, a new type of diffusion process involving the element Beryllium was introduced. Depending on the starting material, many different colours can be achieved with this process which in some instance involves a complete penetration throughout the stone.
When a corundum is found to be treated with such a process, Diffusion Treated sapphire is stated on the Report and an additional footnote detailing the nature of the process is added .