Unusual Graining Structure in Pink Diamond
Natural pink diamonds examined for origin of color determination often display visible pink coloring along planes of stress. This graining is visible at a slight angle to the plane and can occur as a single plane or multiple parallel planes. The color may be either uniform or unevenly distributed along the graining. One pink stone recently examined in the Carlsbad lab was noted to have a uniform plane of pink color with a colorless hole, seen in the image on the left. The diamond was graded Very Light pink, so the colored plane had low saturation, but the irregularity was impossible to miss in the microscope. When examined under polarized light, a cone of strain could be seen intersecting the plane and fitting perfectly in the colorless hole, as seen in the image on the right.
Pink graining is the result of an unknown defect with an unknown formation mechanism (J.G. Chapman and C.J. Noble, “Studies of the pink and blue coloration in Argyle diamonds,” Summer 1999 G&G, pp. 156–157). It seems to be related to the plastic deformation of diamond, evidenced by the color lying along planes of stress. Whether pink graining occurs during diamond growth or is formed after by stresses or heat from the earth (or a combination of the two) is also poorly understood. It is therefore a mystery as to whether the cone of strain formed first or concurrently and prevented the formation of pink graining at the circle of intersection, or if the cone formed afterward and destroyed the color centers associated with pink graining. Until diamond formation is more fully understood, this dynamic interplay of strain will remain a mystery.
