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