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“Basalt is the result of basaltic magma that has flowed onto the Earth’s surface, and so has cooled and frozen rapidly. When this happens, there is little time for the atoms and molecules in the magma to make their way, by diffusion, to the surfaces of growing crystals. Hence, basalt is made of a mass of tiny crystals, which can only be really seen using a microscope—geologists say it is ‘fine-grainedâ€�. If cooling is exceptionally rapid (as at the chilled surface of basaltic magma that has come into contact with water) then there is no time at all for crystals to grow, and volcanic glass forms instead. Such glass is inherently unstable, or more precisely metastable, because the atoms and molecules are in higher energy states than they would be if they were locked into the ordered molecular structures of crystals. Hence, over millions of years—and even in the solid state—the atoms and molecules diffuse, very slowly, the tiny distances necessary to lock onto other atoms and molecules, and they begin to form incipient crystals. The rock thus devitrifies and turns from clear to opaque, as a myriad submicroscopic crystal boundaries scatter the light.”