Imaginary (k(E), top) and real (n(E), bottom) part of the refractive index from fitting photons with energies 0.5 keV < E < 4 keV. The red line indicates the median expected value from the fading of X-ray rings for GRB 221009A. The shading indicates the 1, 2, and 3σ uncertainty regions. The optical constants calculated for different compositions are plotted for comparison (Henke et al. 1993; Draine 2003) with astrodust (Draine & Hensley 2021, with porosity P = 0.2). Individually, Fe, SiC, or C do not match well, while mixtures of Mg- and Fe-silicate grains provide the closest match.