Published: May 2012
Heat treatment, or thermal enhancement, is one of the most common forms of treatment applied to many gemstones. Gemstone treaters have experimented with heating gems for well over one hundred years, but only within the last century have they achieved consistent results, especially within the past few decades. Heat is harmful to many materials including gemstones. The heat can destroy the delicate balance within a gem, so it is only applied to those stones that can tolerate higher temperatures. Heat treatment is often described as continuing what nature started. While this makes sense in some ways, the conditions in a kiln are generally quite different from those where crystals naturally form.
There are many different types of heating. Heat treatment is usually performed in kilns or ovens at temperatures from 200 to 2000 degrees Celsius. Material to be heated is usually packed within a crucible, along with sand or similar material in order to allow the material to heat up slowly and avoid thermal shock. Some kilns allow for controlled environments where oxygen and other air-borne elements cannot reach the stones. Some other methods can be very crude, much like a campfire. The temperature, the duration, and the rate the temperature rises and lowers all vary depending on the material, the purpose of the treatment, the particular equipment and the gemstone treater. Individual treaters will often have their own closely guarded formulas and recipes that work well for them and the particular materials they work with.
Heat treatment can be done with heat alone, or with various additives and conditions. We will discuss the simpler heating in a normal environment first. This is most like heating food in an oven. Stones that are typically heated in this manner include green tourmaline, blue zircon, tanzanite, aquamarine and citrine. At the higher temperature, electrons within the crystal are able to move about and form different bonds. This results in a change of electron valence, which can result in a different color. In both aquamarine and citrine, the iron (which causes the color) changes valence states. In aquamarine, this results in a bluer color, removing any yellow. In citrine, it creates the yellow color from what was a purple amethyst. The science can get quite complicated depending on precise chemistry, but this is the general rule.
Sapphire and ruby can be heated in fairly primitive ways or by a rather complex process, depending on the specific chemistry of the material and desired results. With sapphire, adding other materials to the kiln is usually not necessary or desirable, but various means are used to control the amount of oxygen in the kiln. The presence or absence of oxygen can affect the resulting color. Allowing oxygen to reach the stones results in the lightening of over-dark sapphire, while heating ruby with oxygen results in a purer red with a less purple tint. By heating a purple sapphire with oxygen, the blue component is largely removed resulting in a pink sapphire. Conversely, preventing oxygen can result in an increase in blue coloring, depending on the chemistry of the material treated. Heat can also melt or dissolve certain inclusions in ruby and sapphires, making a stone appear cleaner and often improving color at the same time. At higher temperatures, close to the melting point, the heat can help internal fissures to heal and close up. This is often helped by fluxes added to the formula within the kiln such as in flux-healed rubies.
Often, heating can occur naturally in the earth, so a gemstone laboratory cannot always determine if a stone has been naturally or artificially heated. This is the situation with most aquamarine and citrine gemstones.
If solid materials are added in the heating process and become a part of the gem, it is labeled as a different treatment. This is the case with glass residues, diffusion, hybrid gems and lead glass-filled ruby, where the additional treatment must be clearly disclosed. Simply stated, "heated” is not adequate disclosure in these cases.
Since most gems are packed within sand or other material to protect against thermal shock, it was later discovered that at higher temperatures, certain packing materials could melt. If the stone had natural fissures or surface pits, these melted substances could enter and fill these areas. The result was healed fissures, or filled surface cavities, or both. Some gems, such as ruby from Mong Hsu, Burma, was ideal for this kind of heating with fluxes, as the original material was commonly heavily included with fine fissures that marred the beauty of the stones. Small amounts of a glass-like residue would fill the fissures, improving the clarity, while leaving visible evidence when viewed in the microscope. This material is disclosed as heat treated with residues.
If this type of heat treatment with glass filling is preceded by an acid bath, the resulting treatment can be more extensive. Much of the lower quality ruby currently found in Africa is highly fissured as well as full of impurities, making it unsuitable for use as a gemstone. By first removing many of the impurities in the gemstone by an extended soaking in strong acids and then heating with glass-forming fluxes, the result is a much improved appearance in terms of both color and clarity. The acid bath results in the removal of dark or opaque material,leaving minute fissures or cavities which can then be filled with glass. The original ruby material remains, but the stone becomes a combination of both natural and artificial elements. In some cases, colorants and other ingredients are added to the glass filling. These stones are described as fissure-filled.
One type of fissure-filled treatment is performed with lead glass. The lead glass has a much lower melting point, allowing greater flow of filler into stones that cannot tolerate higher temperatures. The lead glass has a greater brightness than non-lead glass and gives a beautiful appearance to these stones. The drawback is that lead glass is easily damaged during the jewelry repair and cleaning process, requiring special care and handling for these stones. Once damaged, the stones become very unattractive and this is not reversible. We disclose these stones as lead glass filled or hybrid ruby, as they contain a significant amount of artificial material. While these treatments are currently done primarily to ruby, they are also done to other colors of sapphire and to some star ruby and star sapphire.
In addition to risks when at a jeweler’s bench, the lead glass filling is easily attacked with prolonged exposure to weak acids and common household cleaners, creating a whitish surface on the treated areas of the stone. Once this happens, the stone must be replaced, but in some cases it can be re-polished. For this reason, lead glass-filled rubies are very affordable because the treatment improves the clarity and color of less expensive, lower-grade ruby rough. The apparent clarity and color of the final product rivals the beauty of natural ruby at a fraction of the price.