Black stones derived from dark green glass were investigated by the method of electron microprobe X-ray analyzer (EMX) and of electron diffraction. It was verified that black stones were composed of copper, sulfur and small amount of nickel from the results of EMX, and their lattice constants were in good agreement with those of Cu2S from the analysis of electron diffraction pattern. On the basis of experimental results the mechanism of the formation of Cu2S in the melt is presented as follows. (1) Na2SO4 and CuO used as refining and coloring agent respectively are reduced forming Na2S and Cu2O at the presence of C under reducing atmosphere. (2) Cu2S is then produced by the mutual reaction of the two compounds formed.