Micro-World Gems & Gemology, Summer 2017, Vol. 53, No. 2

Mysterious Iridescence in Aquamarine

This aquamarine crystal showed iridescence as it is turned under a moving light source.
Figure 1. This 40.27 ct aquamarine contains eye-visible parallel stringers that could be
responsible for interference colors seen when the crystal is viewed along the c-axis with
a fiber-optic light. Photo by Kevin Schumacher.

Iridescence is a common interference phenomenon seen in many minerals and gems. It occurs when light travels through a stone and interacts with a thin film that has a different refractive index (RI) than the host material, producing a rainbow effect. Iridescence caused by a layered structure occurs on or near the surface of many gems, including iris agate, “rainbow moonstone,” and the fossilized ammonite gem known as Ammolite. Iridescence can also indicate the presence of a fracture or cleavage (see Fall 2016 Micro-World, pp. 312–313).

An appealing 40.27 ct aquamarine crystal on calcite matrix owned by Lucas Fassari (Costa Mesa, California) featured eye-visible, cloud-like stringer inclusions that

extended from the base of the crystal parallel to the c-axis (figure 1). Inexplicably, examining the stone down through the c-axis with oblique fiber-optic illumination revealed a concealed iridescent, slightly three-dimensional “shimmer” that shifted colors as the light source moved (figure 2). What was puzzling was that in the iridescent regions  there seemed to be no evidence of a break, thin film, liquid inclusion, or other discernible feature that would cause these interference colors. We hypothesize that the cloud-like stringers could be creating dislocations, producing a structure capable of generating interference colors in the localized region just above the stringers.

Photomicrographs showing iridescence in aquamarine.
Figure 2. The surface of the aquamarine crystal contains a hidden iridescence that is only visible when the light source is at certain angles relative to the surface of the stone. The angle of the oblique fiber-optic illumination in the photo on the left does not make the iridescence visible. As this light is swept across the surface, changing the angle of illumination, colorful iridescent colors appear (center). The appearance of the phenomenon alters along with the angle of illumination (right). The large dark area in the right portion of each image is a reflective void. Photomicrographs by Nathan Renfro; field of view 4.8 mm.

This aquamarine crystal is one of the most interesting the authors have encountered because of its unusual and unexplained iridescence.

Nicole Ahline and Maryam Mastery Salimi are staff gemologists at GIA in Carlsbad, California.