What is a rock? A purely descriptive definition is that a rock is - A naturally occurring aggregate of minerals and other solid material. - Usually, there are several minerals in the aggregate, though some rocks may have only one. The other materials may include natural glasses, organic material, or fossils.
Geologists usually think of rocks in a second important way, however. Please and recite it like a mantra:
A rock is a record of the environment in which it formed.
Whether those crystals are large or small depends on how quickly they cooled. There are two basic subgroups:
There are three basic types: