Gemstones have been "improved" -- most often by cooking at high temperature -- since antiquity. Most zircons occur in nature as a dull red; heat treatment turns them blue or colorless. Aquamarines are normally found green and heat treatment turns them pale blue. Heat treatment of corundum (sapphire and ruby) is so common that one should always presume it. Disclosure rules are constantly being changed to take account of new variations in heat treatment, but as of this writing, normal heat treatment is classed as a "traditional treatment" so common that it needn't be disclosed to prospective buyers of gems. Do your own search on the trade associations that deal with (so far self-imposed) industry rules of disclosure: CIBJO (Confederation Internationale de la Bijouterie, Joaillerie, Orfevrerie, des Diamants, Perles, et Pierres), AGTA (American Gem Trade Association).

Heat treatment of corundum can change both its color and clarity. The traditional tool is an oil drum heated by a coal fire. People who claim to know how to heat-treat are very secretive about the conditions and ingredients they use. Modern heat treatment uses furnaces (example pictured at right) capable of sustaining temperatures near -- or even above -- the melting point of corundum (2050o C.) for many hours. The variables in the secret recipes are: the duration of the heating and cooling stages the temperatures sustained during each stage whether the atmosphere in the furnace is oxygen-rich (oxidizing) or hydrogen-rich (reducing) the presence in the furnace of other elements intended to diffuse into the corundum crystals. the presence in the untreated corundum crystal of coloring elements unique to its geological origin

Neither the mechanism of heat-treatment nor the various causes of color in corundum are perfectly understood. Gentle heat-treating -- in the range of 400^o C.-- can supply enough energy to dislodge stray electrons in the crystal structure. These "color centers" are thought to be a cause of yellow in corundum. Because the energy required to displace color centers is small, corundum colored in this way can be discolored by long exposure to the sun, or by the touch of a soldering iron, which can reach 900^o C. At the higher temperatures sustained in normal modern heat treating (>1750^o C.), the crystal structure becomes sufficiently plastic to permit bigger objects than electrons -- whole atoms -- to move within it, enter it, or leave it. Color changes brought about at these high temperatures are reversible only at the same high temperatures.

The color blue in corundum is thought to be caused by a charge -- an electron -- slipping back and forth between atoms of two charged impurities, iron and titanium. Normal light supplies enough energy to cause the structure to go from its ground state to an excited state, after which it quickly returns to its ground state with the emission of a little heat. Color happens because only certain wavelengths of light are absorbed in this process, leaving the rest for us to see. The key for the corundum cookbook is the presence of both iron and titanium impurities. A titanium impurity alone causes no color; iron alone causes greens, yellows, and oranges. Only a tiny amount of impurity is needed, on the order of hundredths of a percent. Red in ruby is caused by the presence of trace amounts of chromium, the same impurity that causes green in emerald, only more tightly bound in the corundum crystal structure and therefore sensitive to light at a different wavelength.

The practical difficulty gem cooks face is that is hard to get larger atoms to diffuse into the corundum crystal structure. Cooking corundum in the presence of titanium or chromium tends to coat the surface only. This is futile when cooking rough as the subsequent polishing will remove the color layer. Cooking faceted corundum in the presence of these impurities is called surface diffusion. The extreme heat blisters the polish, requiring a re-polish, which leaves a color layer thicker around facet junctions than on faces. The irregular depth of color is visible when the stone is immersed. Recently gem cooks have apparently had some success diffusing a very small atom -- beryllium -- deep into pink corundum, producing a pink-orange color. This treatment is new enough to require disclosure. It has been called "bulk diffusion."

Large atoms are less reluctant to leave the crystal than to enter it. Extreme heat can have the effect of driving off iron. That is why illumination by ultra-violet light can provide evidence of heat treatment: iron absorbs UV light so the absence of iron permits the gem to fluoresce.

Heat treatment stands the best chance of success when all the necessary elements are already in the crystal. Gueda sapphire from Sri Lanka used to be discarded because it was so full of rutile needles (titanium dioxide crystals) that it appeared cloudy. Neither was it blue. Heat treatment caused the rutile to break down and the titanium to enter the surrounding corundum crystal. If there were also enough iron in the crystal, the charge transfer mechanism described above could be achieved, and with it, a blue color. As the rutile needles were absorbed, the cloudiness disappeared as well.

crucible conglomerate

heat treated Montana sapphire

There are many tests which provide evidence of heat treatment. They suffer from false negatives: the absence of evidence usually does not prove the absence of heat treatment. Since almost all commercial quality sapphires and rubies are assumed to be heat-treated, the gemmologist's burden of proof is to demonstrate the absence of heat treatment and the amount of effort required is justified only for very large and expensive gems.

For further reading:

The best place to start is the Summer 2003 issue of Gems & Gemology: "Beryllium Diffusion of Rubies and Sapphires" by John Emmett, Kenneth Scarratt, Shane McClure, Thomas Moses, Troy Douthit, Richard Hughes, Steven Novak, James Shigley, Wuyi Wang, Owen Bordelon, and Robert Kane.

For the particular chemistry of Montana sapphire, the best account is in the Winter 1993 issue of Gems & Gemology: "Heat Treating the Sapphires of Rock Creek, Montana" by John Emmett and Troy Douthit. Most of the sapphires shown on this site come from Dry Cottonwood Creek, but the chemistry is similar.

The best book on causes of color is by Kurt Nassau: The Physics and Chemistry of Color: The Fifteen Causes of Color Published by Wiley Interscience. ISBN 0-471-86776-4. Very expensive but worth it.