Bulk or Lattice diffusion of Sapphires, is a color enhancing procedure originating out of Thailand. The procedure has generated a disclosure controversy in the world’s sapphire  market.
WHAT IS DIFFUSION?
Diffusion is the introduction of a new or enhanced color into a gemstone of the corundum
family of gems.  The corundum family contains all the colors of Sapphires and Ruby.    THE FIRST DIFFUSED STONES. 
The first diffused stones were blue sapphires.  Light colored sapphires were coated with a titanium film and then heated to a very high temperature.  The titanium melted and penetrated the surface of the stone, creating a nice, intense blue color. In this treatment process, the stones got so hot the surface would actually start to melt and become textured. These deeper colored, almost melted gems had to be re-cut and re-polished to make them marketable.  The re-working process took material and color away from the center of the stone facets and caused a color concentration at the facet junctions, creating a noticeable dark pattern when the gem was viewed from the back.  This skeletal structure was easy to see and provided unquestionable proof of enhancement in blue sapphire by color diffusion.
THE LATEST, BULK OR LATTICE DIFFUSION
The latest, bulk or lattice diffusion is heat treatment of another type it uses less heat over a longer period of time. Using this method, any number of sapphires can be placed in a heating vessel, usually a ceramic bowl or crucible.  The crucible is lined with other materials to prevent any stone breakage from initial heat shock. This heat treatment of sapphires is a standard practice and is accepted in the gemstone industry.  Unknown to the treaters, a piece of chrysoberyl, probably an alexandrite, was mixed in with the lining material in the crucible.  During the heating process the chrysoberyl gave off a beryllium gas that altered the atomic lattice structure of the material being treated.   The altered lattice produced some very intense yellow, orange and reddish colored sapphires, some of these enhanced colors, mirror the best colors found in nature and some of the colors have not been found in nature.  The first batches of the new treated stones were thought to be from a new location and were introduced into the trade as having only been heat treated. These beautiful new intense colors were immediately accepted by the trade and sold rapidly at high prices. The beryllium was discovered after the heating crucible broke and was replaced by a newly lined crucible.  All of the ”new location” stones out of the new heat treating crucible did not have the new or intense colors that the treaters were expecting to see from  the “new location”, they saw instead, stones that looked just like the old location.  The lining of the old, broken crucible was inspected and the chrysoberyl was discovered.  Upon closer inspection and additional experimentation the treaters found out that the chrysoberyl in the lining was responsible for the new colors and there was actually no “new location” at all.  This announcement, when it was finally made public, created a lot of confusion in the trade. There is no simple way to separate the bulk or lattice diffused stones from non-enhanced stones and some dealers are still selling color enhanced stones as natural stones.    
DISCLOSURE AND CERTIFICATION
The dealers who bought some of the first production stones were professionally embarrassed after the artificial color enhancement was announced and refunded huge sums of money to their clients.  Some dealers refused to accept responsibility and took a “buyer beware” attitude. To quote one foreign dealer “we don’t have the equipment to test for it and if we did, we don’t have anyone qualified to use the equipment if we had it, if you think its diffused don’t buy it” Something like if you buy it, you own it.  Almost every dealer or supplier wants a certificate from one of the many testing labs before the stone can be exchanged or returned for a refund.  The large testing labs are using expensive testing equipment to determine if the stones have been enhanced or not and charge prices that reflect the cost of the equipment, sometimes as much as $500 per stone.  Good luck on getting this out of pocket expense refunded by your supplier. This expensive equipment is not practical or affordable for most stone dealers and gemologist, forcing them to use the trade labs on questionable stones.  The only other route to take is to quit selling sapphires in the color ranges that are suspect.  Hopefully in the not to distant future someone will come up with an easy method of detecting bulk or lattice diffused stones, removing these beautiful sapphires from the suspect treatment list and putting them in their proper place as desirable enhanced natural gemstones.