‘Fracture filled’ or ‘clarity enhanced’ diamonds are those whose fractures or cracks have been filled in to restore and enhance their brilliance. The process for fracture filling diamonds was developed in 1982 by Israeli diamond cutter Zvi Yehuda and involves filling surface-reaching cracks with molten glass to improve the diamond’s clarity. Yet, it was not until the 1990s that fracture filled diamonds began to appear in the market in abundance.
The glass used has a high refractive index similar to that of diamond, making the fractures less visible. The treatment is performed under heat and pressure, sufficient to force the liquid glass into the fractures. This method tends to improve a diamond’s clarity by one grade but it doesn’t affect the colour or weight of the gem. Only diamonds with small cracks can be fracture filled. However, it is important to note that many diamonds contain minute internal "cracks", sometimes described as “feathers” by the retailer, that do not pose a threat to the integrity or life span of the diamond.
The process of fracture filling divides opinion among jewelers, with many attesting to their advantages and disadvantages. Yehuda originally said that the advantage to fracture filled diamonds is their lower cost, creating the potential for someone to purchase a larger, cleaner diamond than they otherwise would have been able to afford. However, purchasing a fracture filled diamond doesn’t guarantee a discount as they will be paying the same price for the grading of the stone before the fracture was filled. The only difference being that a visible inclusion has now been made "invisible" to the naked eye. Furthermore, fracture filled stones are often too small or too low quality and are therefore rarely submitted to gemological laboratories. Laboratories will not provide a clarity grade as the enhancement is not a permanent treatment and the stone’s appearance can diminish over time. Subsequently, it is difficult for the buyer to know exactly what they are buying or if the price is right.
Fortunately it is easy to spot a fracture-filled diamond; simply shake the stone from side to side under a microscope and you will notice a ‘flash effect’, a play of bright colour spanning from a purple to an orangey-yellow. If a diamond has been tampered with the colour of the glass can also be a giveaway as a yellow-brownish shade is often made visible in transmitted light, even impacting upon the overall colour of the stone.