Pleochroic halo

A pleochroic halo, or radiohalo, is a microscopic, spherical shell of discolouration (pleochroism) within minerals such as biotite that occurs in granite and other igneous rocks.

The inclusions are typically zircon, apatite, or titanite which can accommodate uranium or thorium within their crystal structures.

[2] Uranium-238 follows a sequence of decay through thorium, radium, radon, polonium, and lead.

(Because of their continuous energy distribution and greater range, beta particles cannot form distinct rings.)

A pleochroic halo formed from U-238 has theoretically eight concentric rings, with five actually distinguishable under a lighted microscope, while a halo formed from polonium has only one, two, or three rings depending on which isotope the starting material is.