Iridescent Inclusion in Tanzanite
Gemmy blue zoisite, more commonly known as tanzanite, is valued for its pleasing violet to blue hue. One tanzanite specimen from Merelani, Tanzania, was purchased by the author for an entirely different aesthetic feature: its well-defined, minimally damaged crystal faces. Microscopic observation uncovered a hidden bounty of color just below the surface, particularly when subjected to metal halide oblique lighting. Crystallographically aligned, vividly iridescent colors resulting from fine separations along the stone’s cleavage planes (figure 1) were visible using fiber-optic light.
Photographing inclusions can be challenging, to say the least. When light enters the host material, each delicately nuanced feature is affected in unpredictable ways. Documenting surface textures or crystal faces is equally difficult; the challenges are manifold when attempting to photograph both simultaneously. Knowledge of lighting techniques is useful in such situations (N. Renfro, “Digital photomicrography for gemologists,” Summer 2015 G&G, pp. 144–150). In this instance, a combination of diffused lighting and direct pinpoint lighting was used to illuminate both features. Low-intensity diffused light was reflected from the surface of the stone so as to illuminate the crystal faces without overpowering the iridescent colors brought from below. Once lit, a short “stack” of images captured the appropriate amount of depth into the stone.