The CdSxSe1−xmicrocrystallites contained in colored filter glasses have been considered to be spherical. This letter demonstrates that in fact they are hexagonal columns, implying a wurtzite structure. Microcrystallite grain growth is observed in real time using an electron beam annealing method. Each grain is confirmed by energy dispersive spectrometry to be CdSxSe1−x. The grain size difference produces different optical transmission spectra due to quantum size effects. On the optical transmission spectrum of a sample with grains of 30 A˚, a new absorption band appears. This band can be explained by size quantization of the valence band.