Micro-World Gems & Gemology, Spring 2016, Vol. 52, No. 1

A Halo in a Sri Lankan Taaffeite

Sri Lankan Taaffeite
Figure 1. This 0.57 ct taaffeite from Sri Lanka measures 5.63 × 4.00 × 3.21 mm. Photo by Elise A. Skalwold.

The lovely chromium-bearing purplish red taaffeite from Sri Lanka seen in figure 1 is home to a swarm of included crystals, but one is particularly intriguing. Due to this crystal’s depth and minute size, it cannot be conclusively identified without destructive testing. But its opaque black appearance leads us to believe it is either thorianite or uraninite. The latter is a uranium-rich mineral formerly known as pitchblende and reported to exist in taaffeites from this locality (see E.J. Gübelin and J.I. Koivula, Photoatlas of Inclusions in Gemstones, Volume 3, Opinio Verlag, Basel, Switzerland, 2008, pp. 649–650).

The strong alpha emissions from such mineral inclusions may lead to stress fractures in the host mineral. Unlike gamma and beta particles, alpha particles are massive and can cause considerable destruction in their path. In this case, it appears the gem has escaped fracturing: the effect is manifested as a concentric two-tiered halo surrounding the black crystal (figure 2). Its three-dimensional bulge-like appearance may be interpreted as radiation-induced distortions in the taaffeite’s crystal structure. These distortions also locally changed the taaffeite’s color, transparency, and refractive index and resulted in other strain features.

Haloed crystal in taaffeite
Figure 2. In the taaffeite, a radiation-induced two-tiered halo surrounds a minute black crystal; the outer tier is subtle. The haloed crystal is most likely thorianite or uraninite, while numerous colorless zircon and apatite crystals are also present. Photomicrograph by John Koivula; field of view 0.75 mm.

While the colorless inclusions display reflective surfaces in oblique light from the right, the haloed black inclusion does not. Instead there is a reflection-like scattering of light from the halo surrounding it. We suggest that this is due to a gradational difference in the taaffeite host’s refractive index farther from the black crystal as light is scattered off of the disrupted region. An everyday example of this comes from observing the sun before it rises above the horizon, an optical phenomenon that would not occur without the gradational refractive index of the atmosphere bending the light. The refractive index of the atmosphere decreases with altitude. Because light rays are bent toward the higher refractive index, we may surmise that the inclusion’s halo has a lower refractive index than the rest of the taaffeite host. To support this explanation, in-depth exploration of the geometry of the halo’s refractive index will be needed.

In the early days of radioactivity studies by scientists such as G.H. Henderson (in a series of five papers published between 1934 and 1939), these inclusions were known as “pleochroic halos.” Although this term may be misleading, the halos’ appearance is partially the result of localized changes in optical properties in the vicinity of the inclusion. Such inclusions are also relevant in determining the age of the planet by studying the characteristics and effects of radioactive guests in various minerals.

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. William Bassett is a research scientist and professor emeritus of geology at Cornell University.