Gemstone Heat treatment

Heat treated natural ruby stones to enhance their color and clarity is almost universally accepted by the gem trade as long as the effect is permanent and it is disclosed to the buyer which effectively usually is not.

Ruby Heat treatment

These days the vast majority of Siamese (Burmese rubies) are subject to heat treatment and even good stones are worked on to make them that little bit better. It has all become very sophisticated and the old method of packing stones in a clay ball and placing them in an oil drum oven packed with coconut husks for insulation is being increasingly replaced by modern gas and electric furnaces specifically designed for the job. 


Now, if the gem would stay like this forever as it is with originals that also would still be somehow acceptable, but it won’t stay like this and that’s cheating. After usually around 3 years the gem deteriorates and what is left is junk one has been bought for lots of money.

Conventional heat "enhancement" is to throw the stone into an “oven” as it was done already in ancient times. Its just like that the secret is to select certain ruby stones before which have visual similarities and heat them up to a certain temperature.

The temperature will make the color and melt away the inclusions to make the gem clear. 


Glass filled is a new treatment to pop up ugly sapphire and ruby stones; this is a newer process because it needs quite some pressure which was not available before. Usually this processing is done with African gems since they lack the basic quality Burmese gems have.





The process is not fool-proof as some rough stones respond better than others, and gem merchants run the risk of damaging or even ruining a gem through careless treatment or overheating. They reckon on a roughly 50 percent success rate if the burning is done by an expert, much less if the person is inexperienced. 

There is no question that heat treatment does enhance the look of a ruby and it is certainly not a fraudulent process in the sense that coloring or oiling (done especially with emeralds) could be. Nothing is being done to the stone that could not have happened in nature. Heat treatment is described by the trade as being just an improvement on nature in the same way as cutting and polishing enhance a gem's appearance. 

Nevertheless, there are differences of opinion over the question. On the one side, there is the indisputable argument that heat treatment does give you a better stone if the process has been done well. The contrary argument does not disagree with this but raises two questions: 'If you are going to improve on nature, where do you draw the line?' and 'What about the competition between a treated stone and in untreated stone?' The first question is less relevant to the ruby since the change brought about by the treatment is less dramatic than in the case of other stones. 

The value of a heat treated Burmese ruby

The red stone's value could be raised on average 0-30 percent by heating. On the other hand, a yellow sapphire for example which is also a corundum stone can be created out of a crystal of extremely poor appearance before treatment. 

The question of competition means a natural stone competing with a treated stone in the market is more thought-provoking since the natural stone is forced to compete with the heat-treated one and, unfortunately, it's no contest the treated one looks better.' The real issue here is that rarity is the biggest single factor in determining a gem's value and a treated stone is less rare than an untreated stone. 

This is a valid point, but so rare are all rubies and so high the market demand that the question becomes largely academic. The pros and cons of heat treatment will probably always involve a degree of controversy, but for practical purposes, it needs to be seen in perspective. Nothing is being added to the stone and somebody could look upon treatments as part of the fashioning of gemstone from their original rough state; they should not be seen in a negative way as long as the treatment process is permanent.' 

While on the question of treatments, it is worthwhile to mention here a substance known as 'ruby oil'. You may get to hear of this red oil being used, but it is nothing to get concerned about. Its only real application is with rough stones as it helps bring up their color. 

It has no relevance to the cut stone as it is merely superficial and rubies unlike emeralds, do not have fractures that would allow the oil to penetrate the stone.The key to reliable shopping for gems is to have faith in your jeweler. Jewelers are a bit like doctors or lawyers; you depend on their specialized knowledge and experience and for the relationship to work there has to be trusted. 

Trust a jeweler?

To build up such trust clearly takes trial and (hopefully not too much) error. You can minimize any risk by using simple common sense when selecting a jewelry store. Look at things like location (in a smart area or not); stock (exclusively jewellery or not); the shop assistants (do they seem knowledgeable, is there a trained gemologist on the staff, are you told a stone's weaknesses as well as its good points or are you given an obvious sales pitch, and so on). 

These and similar common sense indicators can give you a pretty good idea whether or not a shop has a high reputation to maintain; if it has you should get a good deal. There are no guarantees, of course, but your chances are vastly improved and keep in mind you still can ask the seller to visit an independent lab together which YOU choose.

When it comes to judging individual stones, it must be remembered there is no substitute for knowledge and experience, no one can become an instant expert. The basic guide is what is popularly known as the four 'Cs' which means color and clarity the cut and size. 

Various attempts over at least two thousand years have been made to improve the appearance and stability of gemstones. Heat treatment to enhance the color of rubies and other gems plus oiling are used since hundreds of years.

In the last two hundred years inserting resin into cracks under high pressure and synthetic stones were added. The idea is always the same to improve the appearance to get higher prices when selling.

Since they are too sensitive to go through this procedure only a small part found have the clarity and sufficient size to make them usable. All this is somehow nothing but cheating the buyers because after a couple of years this add on material will deteriorate and the pretty thing become worthless.

Especially in Thailand it needs to be very careful since the people usually don’t think what will happen tomorrow. Actually the country is the biggest exporter of this stones and most of this gemstone heat treatment or enhancement is done in Chanthaburi.

Although some treatments have been practiced for centuries (especially heating), modern technology allows more changes as in the past. People draw the line between acceptable handling in favor what brings more money; most don’t care about the customer’s problem after some years. This fact needs attention and being very careful when buying. In every city where the gems are traded are labs to have the stones tested for relatively low prices.

• The current ruby heat treatment process in use is based upon that patented by the Union Carbide Corporation in 1975.

To the normal buyer gem materials such as ruby and sapphire appear to be absolutely solid impregnable fortresses. However, this is but an illusion, for if an object is of small enough size, as with certain atoms, it can pass through a gem as easily as sand through one's fingers. The process whereby atoms may move through a material is termed diffusion and is responsible for a number of the different changes in corundum subjected to heat treatment.
When a crystal is heated, interatomic spacing increases by an amount proportional to the temperature reached, making it possible for atoms to move, or diffuse, through the structure. Atoms of a small size, such as hydrogen or oxygen diffuse rapidly through corundum. Thus, color changes involving these elements; can be effected throughout the entire gem in as little as a few hours by heating gemstones under the proper treatment.

However, the diffusion of transition metal coloring agents, such as iron, titanium and chromium, is far slower, so even heating the gem at 1800°C for several days will produce a movement of only half a millimeter or less diffusion of atoms by means of heat treatment has been applied to rubies and sapphires on a commercial basis for the past several years with the aim of imparting a thin layer of color at, and just below, the surface.

To do heat treatment for a better look, colorless to lightly colored corundum gems are readied for treatment by preforming and cutting, but not polishing, as this inhibits the diffusion somewhat. They are then embedded in a crucible containing a powder that consists of alumina, titanium and metallic oxide colorants. Alumina itself has no effect, while the titanium aids surface penetration and lessens the formation of color bands.

The coloring agents used are generally the same as those employed for doping synthetic corundum. Now the crucible is heated to between 1600 and 1800°C for up to 200 hours. This allows the atoms within the powder to diffuse into the surface areas, creating a thinly colored skin which, after a light repolishing, is rarely deeper than a few tenths of a millimeter.

Thus, natural rubies and sapphires in which the color is too light, or uneven in distribution, can be treated by the above method to correct these deficiencies. Due to the relatively large quantities of clean lightly colored sapphires available from Sri Lanka, this material is most often used for surface diffusion treatments. Stones from other localities may also be used, but less frequently.

In terms of the colors produced, the majority are blues, with some rubies (reds), padparadsehas (oranges) and yellows also being treated. When the process is done properly the colors resulting from surface diffusion can be nearly equal to nature's finest efforts.

However, because the coloring agents have been artificially induced into the gems, and because they are only at the surface, diffusion-treated stones have not been accepted by the trade in the same way as those involving heat treatments alone.

Thus, it is imperative that gemologists are able to identify these stones. A visual inspection of diffusion-treated gemstones yields comparatively little of use with regard to their separation from untreated stones.

Deeply colored pieces may at times appear slightly darker around the girdle, because of the concentrations at the surface.

There is a general agreement that non-permanent treatments such as irradiation, surface diffusion and repair, oiling and dyeing should be disclosed. However some would draw the line at permanent treatments involving heat.

Opponents to the disclosure of permanent gemstone treatments feel that it is unnecessary and will confuse the customer, eventually hurting business. They believe that so long as a treatment is permanent there is nothing to worry about; the whole controversy is so much needless fuss generated by a group of consumer witch hunters.

Is it neatly, though? The most important factor in maintaining the value of precious stones is the rarity. To see that this is true we need only look to synthetic corundum.  A synthetic ruby / sapphire produced by the Verneuil method is a gemstone in every sense of the word, with the same hardness, durability, color permanence and beauty as nature's own. However, because it is produced in almost limitless quantities, its cost is but pennies per carat.
The major factor separating it in price from the natural stone is rarity, and should deposits of natural ruby be discovered which yield quantities similar to the synthetic production, we could expect that its price would also drop to a similar level. 

Fortunately for those who own or deal in natural rubies, this possibility is remote, at best.  The additional quantities of ruby and sapphire produced by treating inferior material also have an impact on supply and therefore price. However, since the amount of treatable rough is limited, this impact is also limited. The heat treatment of yellow and orange sapphires from Sri Lanka was not widely practiced before 1981. Before this, a fine, naturally colored, deep-yellow/ orange sapphire from Sri Lanka of 5 carats fetched $400 a carat, or more, at the wholesale level. Today, the same stone would bring only a fraction of that amount because it must compete against heat-treated Sri Lankan sapphires.

It is unfair competition, to say the least, for the treatment process yields deep-orange-yellow colors with unnerving frequency, hues that were almost unheard of before 1981. If, however, a distinction were to be made between natural and treated gems, it could be sold for what it is: a rare, naturally colored, deep-yellow Sri Lankan sapphire.
Many of those involved in treating or selling treated rubies and sapphires fear that prices for these stones would fall should they be sold with a 'color-enhanced' label attached. This seems unlikely, as supplies of treatable rough are limited. Studies undertaken in the United States by the trade magazine Jeweler's Circular—Keystone have also shown that if consumers are offered a choice of either a naturally colored sapphire or a heat-treated sapphire of similar quality but lower in price, many will opt for the treated stone.

When heat-treated ruby and sapphire disclosure of such treatment are made public it would have note much impact. Instead, the entire gem and jewelry business will benefit from the increase in value of naturally colored gems. Perhaps of even greater importance, by the total disclosure of gem treatment jewelers will no longer have to incur the wrath of a customer who learns that their new ruby or sapphire has more likely than not been heated to improve the color. Gemstone treatments can only help boost sales in the long run actually ruby heat enhancements is already somehow normal today.

The job involves the creation of the most beautiful gem possible and, at the same time, recovering the maximum amount of weight from the rough, an enormously difficult proposition when bearing in mind that each of these ideals can only be achieved at the expense of the other.

Thus, success for the lapidary is measured in terms of an ability to strike a delicate balance between beauty and size. When working with costly materials such as ruby and sapphire, particular attention must be given to retention of weight. Because of pressures to squeeze every last point from the rough, cutters may be forced to retain small surface defects, such as pits and cavities. it is simply a case of the lesser of two evils, for although grinding the defects away completely will make the gem more attractive, this improvement is often e not enough to offset the loss in weight that results. Thus, the presence of surface pits should not indicate a lack of skill on the part of the cutter, but to serve as evidence of the difficulties in turning a profit within this highly competitive business.

Competition is of course vital to the well-being of any business, for it is what fuels the search for the better mousetrap. However, not everyone plays by the rules. During the first six months of 1985, gemologists in both London and Bangkok discovered rubies in which the surface blemishes had been filled in with a transparent colorless glass. Dubbed “surface-repair’ which represented a new method of dealing with an old problem. However, the treatment not only reduces or eliminates the visibility of surface defects, but also increases the weight of the stone at the same time. As a result, it cannot he considered an acceptable treatment, and it is important that the trade is both aware of, and able to detect, this latest charade.

To date, the vast majority of surface-repaired stones encountered have been rubies, with a handful of blue sapphires also turning up. However, there appears to be no reason why gems other than corundum could not be treated in a similar manner.
Details regarding the actual method of treatment were recently supplied to the author by someone who had talked to a man performing surface repair on rubies.

This gentleman from Chanthaburi was primarily involved in the heat treatment of yellow sapphires from Sri Lanka and claimed to be the first person in Thailand to treat rubies by surface repair. According to him, surface repair involves applying a silica-based gel to the surface of cut stones with a paint brush in the areas to be repaired. The stones are then heat treated, turning the gel to glass and fusing it into the pits and cavities in the process. He claimed that his surface repair jobs were superior to all others as the glass adhered tightly to the surface and did not fall out like those of other burners.

It should be mentioned that the idea of filling in surface cavities with a foreign substance is not totally new to the trade. In the past, a number of different materials including epoxy and doping shellac have been used to hide unsightly defects, particularly on cabochon-cut stones. Patents have also been taken out on processes very similar to surface repair, such as those for the glossing of corundum and spinel rods The detection of surface-repaired rubies and sapphires can be readily accomplished by noting the following factors:

I. The difference in luster between the glass filling and the surrounding corundum under an overhead light.
2. The difference in relief between the glass (refractive index 1.52) and the corundum (refractive index = 1.762-1.770) when immersed in methylene iodide.
3. Inclusions of spherical gas bubbles within the glass filling.

4. The colorless nature of the glass filling.

There is no question that heat treatment does enhance the look of a ruby and it is certainly not a fraudulent process in the sense that dying or oiling (done especially with emeralds) could be. Nothing is being done to the stone that could not have happened in nature. 
Heat treatment described by the trade as being just an improvement on nature in the same way as cutting and polishing enhance a gem's appearance. 

Nevertheless, there are differences of opinion over the question. On the one side, there is the indisputable argument that heat treatment does give you a better stone if the process has been done well. The contrary argument does not disagree with this but raises two questions: 'If you are going to improve on nature, where do you draw the line?' and 'What about the competition between a treated stone and in untreated stone?' The first question is less relevant to the by since the change brought about by heat treatment is less dramatic than in the case of some other stones.