The Dictionary of Science defines diffusion as “the process by which different substances mix as a result of the random motions of their component atoms, molecules and ions. In solids, diffusion occurs very slowly at normal temperatures.”
In the Corundum industry there are two enhancement-processes that use the word diffusion: “surface diffusion” and the new “bulk diffusion process.” The latter is also known as E(IM) (Enhanced by Inducing Internal Migration and Formation of Color Centers). Other than the word “diffusion,” the two processes have little in common. This unfortunately seems to be causing many misconceptions about the differences between the two processes.
What is surface diffusion and how does it work?
Surface diffusion relies on external metallic coloring elements being diffused onto the surface of the gem. Compared to the E(IM) method, it produces very consistent results. Generally cheap, pale, or colorless corundum is used as a “blank canvas” upon which desired colors can be diffused or “painted.”
Natural corundum is Aluminum Oxide (Al2O3) colored by metallic elements (Fe, Ti, Cr, etc.). Surface diffusion is achieved by coating these “blank canvases” with Aluminum Oxides and the appropriate coloring agents (Iron and Titanium for Blue, Chromium for ruby, Titanium for Star Sapphire, etc.). The gems are then subjected to high temperatures over a long period of time.
The depth of surface diffusion penetration is small (a fraction of a millimeter). After enhancement the gems need to be re-polished to remove the damage left by partial melting of the gem’s surface. After re-polishing, this very thin layer of color is often distributed unevenly. Facet junctions often are darker due to diffused color penetrating from both sides of the junction. The coloring can also bleed into surface-reaching inclusions and fractures, which are seen as areas of intense coloration.
Clearly seen in this cross-sectioned Diffused Blue Sapphire, is a very thin layer of blue color where the cross-section meets the table facet.
Intense blue color has bled into an open fracture that has reached the surface.
Surface Diffused Gems are very convincing imitators of Natural ruby. They often contain natural inclusions that can fool the unwary. IJ strongly recommends that both jewelers and gemologists keep at least one reference sample in their collections for comparison with natural gemstones.
What is Beryllium Treatment of Beryllium Diffusion
In E(IM) it is thought that “color centers” are the chromophoric catalysts (i.e. the cause of color). The migration theory states that beryllium catalysts flush through the sapphire knocking magnesium atoms out of place. Like billiard balls, beryllium acts like a cue ball to knock magnesium balls away from their original positions to form “color centers.”
So what are the key differences between E(IM) and surface diffusion?
In the E(IM) method, the new colors produced are dependent on each gem’s chemistry. Identical or similar colored sapphires simultaneously subjected to E(IM) enhancement show numerous color variations. In fact, leading scientists and gemologists have been surprised by the variety of colors produced by this method.
By comparison, surface diffusion does not depend on a gem’s inherent chemistry. With controlled quantities and mixtures of coloring agents, surface diffusion has predictable and consistent results.
As individual colors produced by the E(IM) method have their own occurrence rates (or for want of another word, rarity), this has led many people to conclude that the price of these gems should be higher than those enhanced by surface diffusion. Unlike surface diffused gems, some E(IM) enhanced gems can be re-cut without any loss of color. Unlike the coloring found in surface diffused gemstones, the color distribution in E(IM) gems is richer and more varied.
A 2.10 Carat Oval Cut E(IM) sapphire. The gem was chosen for its notable inclusions beneath the table facet.
The gem has been now re-cut into a 1.51 Carat modified Emerald Cut. See the presence of the same inclusions. The stone has kept the same color hues.
The gem cut in half and submersed in Di-iodemethane. Since the coloration reaches the core, this gem could be re-cut without any loss of color.