It was in the mid-1980s that rumors began circulating about diamonds being available in the market in which cracks and cleavages in the stones appeared to have miraculously disappeared. It was then revealed that the individual responsible for developing the process was Zvi Yehuda, an Israeli diamantaire and technology specialist, who reportedly had  refined a process of filling fractures in the stones. Soon after, his company and a number of other firms began selling what they called clarity-enhanced diamonds.

The concept of filling cracks in a gemstone so as hide the presence of fractures is not new, and indeed was known about as far back as Roman times, when the practice of oiling emeralds was first introduced. The principle is relatively simple. If a crack is filled with a substance that has a refractive index close to that of the host stone (in the case of the diamonds it is 2.417), then the path of the light passing though the gem will not be disturbed, and the crack will be virtually imperceptible.

Yehuda revealed little information about his process of fracture filling, but it was suggested that the filler material was a compound containing lead, chlorine, oxygen and bismuth, which was injected into the stone  under pressures  of about 50 atmospheres and temperatures around 400 degrees Celsius. Other companies were reported to be using higher temperatures. A report by the Gemological Institute of America (GIA) noted that a vacuum chamber may be involved. Later, it was reported that alternative filler materials involved silicone-type substances.

An in-depth study conducted by GIA in 1989 reported that, while fracture filling was relatively stable under normal handling conditions, when the diamonds are placed under certain types of stress, the treatment can be reversed. One these is boiling diamonds in sulfuric acid, which is a process sometimes used to remove metal remaining from the manufacturing process from the surfaces of rough girdles, and to clean out laser-drilled inclusions. It also was reported that, if a stone is repolished or recut, the heat generated by the process  can result in the filler being lost entirely. If jewelry is repaired without the diamond being removed from its setting, the filler material is also likely to be damaged or lost. GIA also reported that the fracture filler material, while helping improve the clarity of a diamond, is inclined to reduce its color grade.

Because it deemed the fracture filling to be non-permanent, GIA declined to issue full grading reports on diamonds it has judged to be fracture filled. The only document provided will be an identification report stating that the stone in question is a diamond and that it has been filled.

The GIA report also pointed out a number of tell-tale signs that indicate that diamond has been fracture filled. These include:

The flash effect, “which is seen when a treated diamond is tilted back and forth, and the color changes from orange to blue and back to orange again in a flashing manner

Bubbles in the filler compound, which are which are small aggregates of trapped gas in the filling compound, generally visible only under microscopic examination.

The flow structure of the filler compound, which under microscopic examination are areas with a glassy,  unnaturally melted appearance.

A crackled texture, which is less common and involves a crackled or web-like texture in a surface reaching the filled fracture.

Incomplete filling at the surface, appearing as extremely shallow areas of incomplete filling at the surface of the stone.

Cloudy surface or filled areas, which are believed to be the result of residue from the treatment process.

The ‘flash effect,’ in which transparent filler material introduced into cracks within a diamond can be detected by a colorful rainbow flash, visible when the stone is turned, is indicative of fracture filling, a clarity treatment.