Spherical or elliptical regions up to 40 micrometers in diameter in which there is a change in color from the surrounding mineral when viewed with a petrographic microscope. Pleochroic halos are found around small inclusions of radioactive minerals—for example, zircon, monazite, allanite, xenotime, and apatite—and in rock-forming minerals, principally quartz, micas, amphiboles, and pyroxenes. Halos have also been identified in coalified wood preserved in deposits on the Colorado Plateau. These small regions of discoloration were recognized by H. Rosenbusch as early as 1873 in cordierite, but it was not until the early twentieth century that J. Joly ascribed pleochroic halos to the effect of irradiation originating from the inclusions.