What Is Natural Stone
SEDIMENTARY ROCKS are made when pieces of rocks settle from the water or when minerals are deposited by plants, animals, and other chemical processes. [e.g. limestone, sandstone, shale]
METAMORPHIC ROCKS are old rocks that have been squeezed and heated but not melted. What new rock is made depends on what the original rock was and on the amount of heat and pressure. [e.g. gneiss, slate, quartzite, marble]
IGNEOUS INTRUSIVE ROCKS are made when molten rock cools while still underground. [e.g. basalt, granite, gabbro]
IGNEOUS EXTRUSIVE ROCKS are made when molten rock flows on the land surface or is thrown into the air and then is cooled into rock. [e.g. basalt, scoria, tuff]
– How to Identify Rocks by Bill Langer of USGS