Stars Are Out in Paraíba Tourmaline
The rarity of Brazilian Paraíba tourmaline, with its vibrant, almost unearthly colors, makes it a coveted gem the world over. What a delight to discover a pair of stars shining out from among many non-phenomenal stones in a parcel. This was a fruitful result of gemologist Elaine Rhorbach’s regular use of a point-source light for examination of all materials she encounters in the marketplace. Wherever she is in the world, this former nurse carries her powerful otoscope—no longer examining ears, but rather checking for any optical phenomena in gems, even where none is expected.
The phenomenon of asterism is seen in a variety of gem materials fashioned as cabochons. The multiple chatoyant bands of light forming these stars are caused by the scattering of planar light by sets of parallel structures. These are often inclusions such as minerals, voids, and structural defects, but they may also be manifested by natural surface features of the crystal left intact on the gem’s base (e.g., tourmaline prism face striations) or etched into the surface by hand. The crystal structure of the host dictates the number of bands, and with mineral inclusions in particular, the orientations are a result of complex chemical interactions between the inclusions and their host. Orientation of the cut gem also determines what effect is seen. For instance, multi-star quartz is a member of the same trigonal crystal class as tourmaline; it displays four- and six-ray stars, depending on viewing orientation in relation to its crystallographic axes of symmetry.
In the case of the 1.06 and 0.90 ct Paraíba tourmaline cabochons seen in figure 1, the c-axis of the larger gem is along its length and that of the smaller specimen is across its width. Thus, the view from above is perpendicular to the crystal’s c-axis. Fluid-filled inclusions paralleling the c-axis dominate the gems’ microworld, causing a distinct chatoyant band perpendicular to the axis. A faint band seen crossing this appears to be caused by the slight widening of the fluid-filled inclusions (figure 2). Since the inclusions are roughly rectangular as they are aligned parallel to the c-axis of the crystal, they produce two directions of reflections oriented at 90°. The elongate direction has more reflective surface area and therefore produces a brighter chatoyant band, while the width has less surface area and produces a weaker chatoyant band from the same reflective inclusion set. While EDXRF detected the presence of copper in these vibrant gems, affirming their Paraíba nature, the subtle stars elevated them into the class of rarefied connoisseur gems—another example of the power of inclusions!
Elise A. Skalwold, an accredited senior gemologist of the Accredited Gemologists Association (AGA), is involved in curating and research at Cornell University in Ithaca, New York. Nathan Renfro is the analytical manager of the gem identification department and analytical microscopist in the inclusion research department at GIA in Carlsbad, California.