13.2: Heat Treatment

Heating of Precious Stones

Following is a discussion of one of the oldest methods for gemstones enhancement.

By Alberto Scarani and Paolo Minieri

In previous articles, we have discussed how adding Beryllium with high-temperature treatments generates a change and improvement of color in corundum. Similarly, we have discussed several aspects of surface diffusion treatment, a modified system, by addition of foreign elements which become chromophores used for altering and embellishing gemstones: the heat treatment.

Before we move forward it will be useful to focus upon an essential aspect: the simple heating of a gemstone can greatly improve its appearance due to chemical changes in elements already present within the stone, or sometimes a combination of elements, when heated, can aid in the "healing" of fractures. The "healing of fractures is very important to distinguish from other heat enhancements methods, as it is more penalized when noted in Gemstone Identification Reports issued by major labs because foreign elements are often added. Truth be told, as we shall see ahead, even with simple heat treatments there are chemical exchanges between the gem and the surrounding atmosphere, but most labs consider this of minor significance.

It’s practically impossible to establish a precise dating of the first gemstones heating experiments. Red agates and carnelians revealing evidence of heat treatment were produced in India in 2000 B.C.; many examples were also recovered from Egyptian tombs, including treasures found with Tutankhamen (Circa 1300 B.C.). In more than 4000 years, the procedure hasn't substantially changed! This will be apparent when you have a look at some elementary ovens that are still operational nowadays in the Chanthaburi area, Thailand and Cambodia. These ovens still produce at full speed heated gems of nice quality.

In the 19th century, the improvements in experimental sciences resulted in continuous upgrading of technical resources, allowing the development of ovens able to reach and easily maintain temperatures of 1500 °C or more. Nevertheless, we must say, that until the first studies were performed by Dr. G.O. Wild in 1932, the effectiveness of heating gemstones couldn’t be reliably estimated even by the most expert “burners”.

But How is common heat treatment performed? Although there are many variations of the procedure and a number of gems are involved, we can summarize by saying that there are two main elements involved: the temperature and the environment (atmosphere composition and pressure) in which it took place. The mechanism which allows the modifications is unleashed by raising the temperature. In the majority of cases, this allows chemical exchanges at atomic levels between the gemstone and its surrounding atmosphere.

There are many different effects which can occur when a stone is heated: from darkening to a color change/improvement, from a structural variation to a crack. It’s clear that the main purpose of a treatment should be to improve the appearance of the gemstone. Therefore, the most interesting aspects will involve color and clarity modifications.
Color modifications can occur:

• in diamonds as the result of with irradiation, annealing
• In varieties of sapphire (adding or reducing color in yellow, green or blue hues)
• Ruby (modifying from browny or purple to red, from red to orange, and removing undesired color components)
• Aquamarine (modifying from green to blue)
• In other beryls (from yellow to colorless, from orange to pink)
• In endless quartz varieties (from amethyst to smokey quartz, cryptocrystalline quartz like carnelians, tiger eyes etc)
• Spodumene and Kunzite (from purple to blue, from green to pink or violet)
• Topaz and tourmalines (almost all of them abide the treatment)
• Zircon (from browny to reddish or blue, from green to blue or yellow)
• Tanzanite (a variety of zoisite) is an emblematic case: it originates in nature, an undesirable brownish-yellow color, only after the heat treatment those stones will get the worldwide-known beautiful purplish-blue color.

The most impressive clarity modification due to heat treatment occurs in sapphire and ruby corundum varieties. The technique required for color modification/improvement that is needed is very similar to what is needed to achieve clarity modifications. It involves proper and controlled composition of the atmosphere. A reducing or oxidizing environment (higher or lower oxygen percentage) will allow, as an example, the exsolution of the rutile needles band of silk with consequent improvement of the transparency in milky, whitish or cloudy stones. The procedure could enhance, reduce or eliminate the asterism phenomena visibility. Exposing gemstones to high temperature often causes distinct modifications. Inclusions (several are often present) could suffer for distinctive alterations that, sometimes (rarely) could lead to irreparable damages. The different dilatation coefficient between the stone and included crystals often produce discoidal fractures surrounding them called tension halos which are very common to find in heated stones.

Another example of diagnostic evidence is the morphological modification of the included crystal shape; well defined, with straight surfaces and distinct edges in unheated stones, they will appear blemished, more rounded and free from clear edges in treated stones, in a direct proportional grade to the reached temperature.

Not always reaching very high temperatures is necessary. Considering the treatment timing length a variable connected with temperature (they’re inversely proportional) we will be allowed to lower the heating (500-700 °c) if we can protract the treatment length. In this way, we will be able to drastically reduce heating effects to inclusions. The low-temperature heating is really difficult to spot by visual observations only, even for trained and skilled gemologist, very often the use of advanced apparatus like FTIR Spectrophotometer et al. is required. Nowadays heat treatment is widely accepted (if disclosed, of course) and in some cases (tanzanite, tourmaline, zircon, quartz etc..) is considered such a consolidate routine that very often the disclosure is unmentioned (among professional) in business procedures. It’s good to know that in those described stones, only rarely the treatment will be considered of discriminative importance from the price point of view, in many other situations (in sapphires and rubies, for example) the importance of this factor is absolutely unavoidable. Nowadays many mines providing for high-quality gemstones are sterile or in the way of depletion while demand for such gemstones remains very high. This has led to attempts to make profitable low-grade materials by using enhancement treatments. It’s the typical case of the so-called “Geuda” Sri-lankan sapphires (the name comes from singalese word used to describe grey-bluish low grade material) flooded the market in the second mid of 70’ and of Mong-hsu Burma rubies appeared in huge quantity on the market in early 90’. Geuda sapphires are commonly near-colorless and the clarity is heavily affected by the abundant presence of rutile in clouds or silk bands shapes, this is in common with Mong-hsu rubies which have almost always distinct blue zoning affecting the color. In both cases, heat treatment allowed a terrific improvement of color (enrichment in Geuda sapphires, photo 10-11 and removing of the blue component in Mong-hsu rubies, foto 12) and clarity by silk (rutile) exsolution. Truth to be said, in Mong-hsu rubies treatment the adding of a foreign component (flux) was needed to permit the healing of the numerous fractures typical of that rough.

We can state without a doubt that the majority of the gemstones on the market are submitted to heat treatment of variable intensity on a regular basis (even if of minimum impact as in some aquamarine cases et al), without appreciable depressive effects on the price side which could be nevertheless considered far greater when the intensive use of foreign elements and/or other manipulations are performed.

Generally speaking, the demand for unheated gemstones is continually rising (In the USA, Japan and in some more frost European markets) principally due to their rarity but even for their incomparable beauty.
But, what about heated stones prices comparing to unheated? Just to have a clue, for a 1 ct unheated Sri-Lankan sapphire there’s an average premium of 20-25%, things will significantly change if we go over 2, 3 or more cts, the price will be doubled, treble and much more in some particular cases.
And what about the Italian market? We are far late, as usual, but the trend shows a distinct and raising appreciation for unheated gemstones and for sure treatment-free will be more and more requested.

Temperatures for Individual Gem Treatments:

Aquamarine: 450 degrees Celcius changes the valence state of the iron
Amethyst to Citrine:: 450 degrees Celcius
Reddish Brown Zircon to Blue: 1000 degrees C in a reducing (oxygen free) environment
Reddish Brown Zircon to Colorless 900 degrees C in air
Orange Beryl to Morganite: 400 degrees C
Yellow and Brownish yellow topaz to Pink: 550 degrees C
Topaz (all colors) to Colorless:1000 degrees C
Dark Green Namibian Tourmaline will brighten Significantly:370 degrees C
Brownish Green Zoisite to Tanzanite:370 degrees C
Sapphire containing titanium to develop or brighten color:1600 degrees C

Bibliography and References

• “Gemstone Enhancement”, Kurt Nassau, Butterworths, London, 1984, ed IGI 1989
• “Ruby & Sapphire”, Richard W. Hughes, RWH publishing, 1997

(Copyright Preziosa Magazine, 2009 - Translated and revised by Alberto Scarani & Barbra Voltaire)

• "Gemmology", Peter Read