Most commonly, minerals occur as large, solid masses in the earth's crust that are called rocks. In contrast to minerals, which are classified by composition and structure, rocks are divided into three groups on the basis of their origin. 

1. Igneous rocks are formed from the cooling of molten materials either at the surface of the earth or at depth. 

2. Sedimentary rocks are formed at the surface under low pressure and low temperature conditions, from materials derived either from pre existing rocks or from organic sources. 

3. Metamorphic rocks are formed at depth, and are rocks that have been altered by heat, pressure, and fluids, without having been remelted. 

Each rock represents a unique environment of formation, and if we understand its characteristics, we therefore understand its formation and origin. And, in turn, we can do much to explain past conditions on this planet.