Fracture-Filled Diamond with "Rainbow" Flash Effect
A loose diamond submitted for a diamond grading report is evaluated and given a grade on a scale relative to each value factor. Clarity, defined as a diamond’s relative freedom from inclusions or blemishes, is graded on a scale of Flawless (least included) to I3 (most included). The most common inclusions in diamond are crystals, which are contained entirely within the stone, and fractures—also known as feathers—which are surface reaching. Various clarity treatments exist to mask or remove undesirable inclusions because high-clarity diamonds are considered more valuable. Some diamond clarity treatments are permanent, while others are not and may change drastically in appearance with time or improper care. For this reason, GIA does not issue grading reports for diamonds that have been treated with unstable, non-permanent treatments. Acceptable treatments such as laser drilling and internal laser drilling are always clearly disclosed on grading reports.
One such unstable treatment is fracture filling. Large, deep fractures in a diamond will often appear white or reflective due to the difference in the refractive index of the diamond and the air within the fracture. In an effort to disguise this, a substance with a refractive index similar to diamond’s, such as highly refractive lead glass, is introduced into surface-reaching fractures to reduce the interference of air (figure 1). Several telltale signs, such as the flash effect, typically reveal the presence of a filled fracture. The classic flash effect displays a vivid pink color in darkfield illumination and a complementary green color in brightfield illumination (figure 2). In a different orientation in brightfield lighting, a filled fracture can also appear bright blue.
A 1.21 ct near-colorless square modified brilliant diamond was recently submitted to GIA’s Carlsbad laboratory for a diamond grading report (figure 3). It was rejected for grading after examination revealed a large filled fracture. Interestingly, the flash effect looked noticeably different from the classic appearance described above. Rather than flashing a single color in darkfield lighting and a single complementary color in brightfield lighting, this filled fracture flashed multiple colors in darkfield and multiple complementary colors in brightfield (figure 4). The unusual effect may be due to a difference in ingredients used to manufacture the filling material. To the untrained eye, it could easily be mistaken for the natural iridescence very commonly seen in unfilled fractures. However, such iridescence typically appears when the viewing angle is nearly perpendicular to the fracture, while the flash effect is most visible when the viewing angle is nearly parallel to the plane of the fracture. In addition to the “rainbow” flash effect, this filled fracture showed a dendritic pattern at the fracture opening that is caused by air pockets in the filling substance (figure 5). This pattern is characteristic of incomplete fracture filling (J.I. Koivula, The MicroWorld of Diamonds, Gemworld International, Inc., Northbrook, Illinois, 2000, pp. 106–108) and provides further evidence of the treatment.
