Some of the green hues are very attractive, and in Ulster County, New York, an unusual shade of blue sandstone was quarried. Granite is an igneous rock, formed by volcanic action. When molten rock, called magma, is forced from the earth's fiery core to the surface, the cooler temperature may crystallize it into granite. There are other igneous rocks, formed by other materials crystallizing in the magma, but granite is one of the best suited for building. This slow cooling process also produces intriguing geodes, semi-hollow granite rocks with beautiful crystal centers.
Marble is the result of an unusual natural phenomenon.
It is classified as a metamorphic rock, transformed from limestone into the soft, malleable stone used for some of the world's finest sculptures and buildings. After extraction from the earth, it dries from its natural state to a hard dense material that can be polished to a brilliant shine. The unusual part is the change in texture sometimes caused by molecular rearrangement, although this transformation also can occur during a change in composition as other minerals intrude. The same process transforms shale, the inferior gray mud and clay building material, into slate, which is commonly used for pool tables. The westward expansion of the American people created demand for every kind of building stone, and quarries supplied it.