Ceylon alexandrite (BeAl2O4:Cr3+) color-change physics: chromium absorption band and transmission windows?

The dramatic color-change phenomenon in Ceylon alexandrite (chrysoberyl / BeAl2O4:Cr3+) is governed by trace chromium substitutions within the octahedral aluminum crystal sites. Chromium creates a broad, intense optical absorption band centered in the yellow-orange region around 580nm, leaving two transmission windows: one in the blue-green spectrum (490nm) and one in the deep red spectrum (above 650nm). When illuminated by natural daylight (rich in blue-green wavelengths), the stone transmits green; when illuminated by incandescent tungsten light (rich in red wavelengths), the red transmission window dominates, shifting body color to brownish-red.