process by which glassy (glass) substances change their structure into that of crystalline solids. Most glasses are silicates (compounds of silicon, oxygen, and metals) in which the atomic structure does not have the repetitive arrangement required for the formation of crystals. Glass is formed by the cooling of a rock magma too rapidly for this structural regularity to become established. Glasses typically are not stable at low temperatures, however, and a readjustment of the atomic arrangement may take place to form more stable structures. This devitrification process is very slow, but over millions of years, a glass will form a completely crystalline mass; thus, the occurrence of very old glassy rocks is rare.