Treatment of Ruby and Sapphire Gems

The mineral corundum also known as aluminum oxide, is one of the best known of all gems.  When pure it is colorless but it is often colored by the presence of various transition metals and color center crystal imperfections. When found in a red color, the mineral corundum is known as Ruby this is usually caused by the presence of the element chromium.  All colors other than red are known as Sapphire.

A large variety of heat treatment processes are used in processing corundum.  Oils, glasses and other fillings are also sometimes used, especially in Ruby. Occasionally irradiation is used, but most irradiation induced corundum colors are temporary and quickly fade in the sunlight.  The most common radiation-induced color is a yellow to gold tone.

Heat to a temperature between 1500 and 1700° centigrade in an oxidizing atmosphere tend to induce a moderate to deep yellow color in pale sapphires.   In deep dark blue sapphires, the same process tends to lighten these dark blue sapphires to a depth of coloration which is more desirable.  This occurs when iron is converted from the plus two valence to a plus three state.  In dark blue sapphires it may also induce the same conversion at times adds a greenish tone when the yellow is added to the blue. The colors produced by these processes is stable and permanent.

Heat to a similar temperature of between 1500 and 1700° centigrade in an reducing atmosphere tends to induce the desirable medium Sapphire blue color in many pale colored sapphires.  A number of Sapphire deposits produce such pale stones, including some of the Sapphire deposits in Montana.   Much more attractive, well colored stones can be created in this process.  It can greatly increase the value of the undesirable pale colored stones. The colors produced by these processes is stable and permanent.

Heat between 1600 and 1900°C for an extended period of time will actually induce silk in stones which contain sufficient titanium in their chemical make up. During this extended heat, the titanium oxide precipitates out as tiny needles which create the star stone effect.  This can turn an otherwise clear stone into a star sapphire Ruby.

Heat between 1600 and 1900°C for a brief period of time will allow some types of inclusions to be reabsorbed or re-dissolved into the crystal structure.   This type of process reduces silk and can eliminate some types of inclusions, including hairline fractures. It is most commonly practiced with Ruby.

Additionally, there are several heating techniques which infuse additional new elements into the corundum Crystal structure.  The first of these used heat to add titanium oxide, in order that a Sapphire blue color could be created.  Generally these impregnation's are very shallow and penetrate the surface of the crystal only a few thousandths of an inch of thickness.  More recently processes have been developed to add beryllium to the crystal structure inducing and beautiful padparadscha pink orange coloration.

The treatment of Ruby and Sapphire is a delicate art requiring considerable practice and effort a few operators have taken the time to learn all the techniques that have mastered this art.

Finally, many rubies and some sapphires are treated with glasses or oils to hide cracks and flaws and improve the apparent clarity of the stone. Unfortunately, the materials which are used to fill the voids are not nearly as strong or stable as the gemstone itself.  Stones which were weak through the presence of cracks and other flaws do not become appreciably stronger through the process of filling them, and any structural weakness that was originally present still remains.  Because the colors produced by the normal heat treatment processes is stable and permanent, they are normally not disclosed to the buyer. However, all elemental infusion heat processes, including the ones used to produce sapphire and padparadscha colors must be disclosed as they are thin and subject to potential removal during repolishing of a damaged stone. They are also considerably less valuable than the natural stones of those colorations.