Rock that forms when sedimentary, igneous, or other metamorphic rock is heated and/or squeezed. Most metamorphic rocks form deep inside the Earth where heat and pressure are intense enough to change the shape of mineral crystals and even change one group of minerals into another.

A volcanic rock that forms when lava solidifies so quickly that there is not enough time for crystals to grow.

A rock or mineral whose metal concentration is high enough to make it commercially viable to extract the metal.

A rock formation that is exposed on the earth's surface.