Micro-World Gems & Gemology, Spring 2020, Vol. 56, No. 1

Phenomenon Resembling Play-of-Color in Sapphire

Angular milky cloud shows spectral colors.
Figure 1. Left: A fine, angular milky cloud as seen in darkfield illumination. Right: Brightfield illumination of the same cloud reveals fine transparent graining causing diffraction of light into spectral colors. Photomicrographs by Britni LeCroy; field of view 3.57 mm.

A phenomenon resembling play-of-color was seen in a 4.13 ct unheated sapphire recently examined by the author. When tilted in darkfield lighting (watch the video below), the sapphire displayed this noteworthy effect, which is most commonly seen in precious opal. It occurred within a small section of a single fine, milky cloud with an unusual shape (figure 1, left). The cloud also possessed extensive transparent graining confined within its borders, best seen in brightfield illumination (figure 1, right).

Phenomenon resembling play-of-color revealed while tilting the sapphire.
Figure 2. Subtle tilting of the sapphire, as shown in this image sequence viewed with darkfield illumination, revealed a play-of-color phenomenon within the milky cloud. Photomicrographs by Britni LeCroy; field of view 14.52 mm.
Sapphire with effect resembling play-of-color

Some regions of the cloud displayed an appearance of rainbow graining with spectral colors confined to the linear grain lines and looked one-dimensional. Other regions, however, expressed a soft billowy broad flash of altering colors with diffuse edges resembling play-of-color and did not follow linear grain lines (figure 2). Play-of-color in opal is defined as a display of spectral colors due to the diffraction of light as it passes through organized, submicroscopic spherical particles. It is possible that the combination of fine milky particles and abundant transparent graining within the cloud were able to diffract light in a way that resembles play-of-color.

Britni LeCroy is a staff gemologist at GIA in Carlsbad, California.