A considerable amount of quartz crystals were found in devitrification products of a soda-lime-magnesia glass used for bulbs of electric lamps or receiving vacuum tubes.Devitrified specimens of a soda-lime-magnesia glass of the composition: SiO2, 67; Na2O, 19; CaO, 7; MgO, 4; Al2O3, 3wt.%, which had been repeatedly heated under its liquidus temperature in block joints of a tank furnace in which the glass was melted, were collected during repair of the furnace. Well developed quartz crystals of surface devitrification had an appearance of the high temperature modification. There were several cases of coexistence with one or two of other minerals such as tridymite, devitrite and sodium disilicate as expected from the phase diagram of the system Na2O-CaO-SiO2, two cases of them corresponding to tridymite-devitrite-quartz triple point and devitrite-sodium disilicate-quartz eutectics respectively. Furthermore albite and diopside were observed with quartz in some specimens.