Published: June 2012
Diffusion is one of the newer treatments applied to gemstones. It is the process of adding elements into the actual molecular structure of a gem. It is different from dyeing or stabilizing in that it is on a very basic, elemental level. The added color cannot be seen with a microscope. There are several types of diffusion, based on the particular chemistry involved.
Surface diffusion is an enhancement method in which certain natural elements that cause color, such as chromium, iron or titanium, are added during the heating of the gemstone. The added elements are the same as those that naturally create the color in gems, thereby adding a natural-looking color. Commonly used on corundum, the added elements diffuse into the material, creating a shallow layer of penetration. Surface diffusion typically penetrates less than one millimeter into the surface of the stone. The stones to be treated are first cut, then put in a crucible along with the color-causing elements, and placed in the kiln. The temperature is brought up to near melting point, in order to allow the elements to diffuse into the gems. Higher temperatures and longer cooking time result in deeper color penetration, but in most cases this is no more than half a millimeter. The diffused layer can contain coloring elements or elements that create asterism - or both. Surface diffusion can create fine colored blue sapphire through the addition of iron and titanium, star sapphire through the addition of titanium oxide, and ruby through the addition of chromium. All of these elements are added to the stones with the addition of corundum powder. Stones are cut prior to treatment and lightly re-polished after treatment, as the high temperatures mar the surface. A gemologist is able to detect this treatment with proper equipment and training, but it is not detectable with just the eye. This treatment is quite stable, but surface diffused sapphire should be handled with special care because the layer creating the effect is only near the surface. A diffused gem may lose its beauty if scratched, polished, re-cut, or otherwise damaged.
Lattice diffusion, also called bulk diffusion, is far more difficult to detect, and unless disclosed, requires confirmation from a lab. In lattice diffusion, ruby and sapphire of various colors are heated at very high temperatures with beryllium. The beryllium can be in the form of a raw powder (beryllium oxide) or in the form of a beryllium-rich mineral such as chrysoberyl. Beryllium is an element that is a component of many gems such as emerald and alexandrite, but is usually not found in any significant quantity in natural sapphires. Due to the very small size of the beryllium atom, it is able to penetrate much deeper into the stones in far less time than the larger elements used in surface diffusion. Lattice diffusion-treated stones require no special care. In most cases, the color penetrates deep enough that re-polishing is not a problem. The longer the treatment is performed, the deeper the color will penetrate. Temperatures used are very high, near the melting point of corundum, and heating times vary from short (just surface penetration) to a week or more (very deep penetration). It is usually performed on faceted gems, but can be performed on rough as well.
Beryllium diffusion affects color at the elemental level in more than one way. It can create a yellow or orange color in a variety of shades, depending on the stone’s chemistry. It can also interfere with the natural color present, removing blue color and thereby lightening over-dark blue sapphires. By the introduction of a yellow color component, various colors of sapphires and ruby can be altered in creative ways. The resulting color is very stable and requires no special care.
Channel diffusion is another type of diffusion that mainly applies to feldspar, andesine-labradorite in particular. This treatment was remarkable for the size of element that was able to diffuse into a crystal lattice. While beryllium is atomic number 4 on the periodic table, copper is number 29 - huge by comparison - and not easily diffused into a crystal. The unique crystalline structure of andesine-labradorite allowed the copper to diffuse along channels, deep into the stone. Research is ongoing, but the basic chemistry is now understood. It does not require special care, but detection is extremely complex as well as destructive, because the artificially diffused material is so similar in every way to the natural. The best treatments, like the best synthetics, will imitate nature. Better understanding and detection of pipe diffusion is the subject of current research. As the diffused material is chemically identical to the natural and current testing is destructive, it is currently to be assumed that all are treated unless natural provenance is known.