Rainbow Graining in Diamond
One of the more subtle inclusion scenes gemologists sometimes observe in natural diamonds is the presence of crystallographically oriented structural defects known as “internal graining.” Often resulting from octahedral and cubic growth sectors of a diamond crystal competing for space during growth, these structural disconformities present as colorless, whitish, reflective, colored, and rarely rainbow varieties of graining (J.I. Koivula, The Microworld of Diamonds, Gemworld International, Northbrook, Illinois, 2000). Colorless graining is the most common type encountered, while whitish graining is somewhat less common and shows, as the name implies, a hazy white appearance in the immediate area of the structural defect zone. Colored graining often occurs when plastic deformation along the cleavage direction results in color causing defects in the diamond crystal lattice. If the defects are numerous enough, they can impart a bodycolor, which in the case of pink and red diamonds greatly enhances their value. Reflective graining occurs as a result of excess strain buildup from the crystallographic defects. The diamond’s accumulated strain is released by separating along cleavage planes, leaving a reflective, mirror-like separation as the most obvious characteristic of this type of graining. The rarest of all types is known as “rainbow graining.” In these remarkable stones, the structural defects in the diamond’s crystal lattice are ordered layers, which act as a diffraction grating to reveal a vibrant display of spectra colors (Summer 2007 Lab Notes, p. 155).
Recently, the author had the opportunity to examine a spectacular example of a yellow diamond containing prominent rainbow graining (see video above). When the stone was rocked and tilted, the rainbow colors would appear and disappear. The elusive beauty of this type of graining and the difficulty with which it is observed make this type of graining a rare and welcome sight in natural gem diamonds.
