The metamorphic rocks are originally igneous or sedimentary rocks that have undergone pronounced alteration under great pressures and at high temperatures. An existing metamorphic rock may also be subjected to such conditions again, whereupon it will be altered a second time. The rocks do not actually melt in this metamorphic process, but their mineral composition is altered, or at least their original texture is disturbed, so that they typically become oriented or even schistose in structure.
Metamorphic conditions usually exist somewhat deeper in the earth’s crust than those required for the fusion of sedimentary rocks, so that it is quite possible for the lower layers of a thick sedimentary deposit to be located at such a depth that the newly formed rock immediately becomes metamorphic. A fine-grained sedimentary rock that has been pressed down to a great depth, so that it is exposed to a high pressure and temperature, may begin to form scattered large grains and the mineral composition of the original sedimentary layer may begin to alter.
The main metamorphic rocks used for building purposes are marble (metamorphic limestone or dolomite), various migmatites, gneisses and schists, and soapstone (steatite).