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Formation
The processes by which rocks and minerals form are different. Minerals form by crystallization and remain true to their nature. Rocks are constantly transforming into other rock types.
For minerals to crystallize, the initial structure of atoms must remain organized despite aggressive exposure to geologic processes. Minerals form in moisture-rich cavities in various types of rocks and grow according to 14 basic crystalline patterns.
Rocks are subjected to a cyclical process during formation. A compound can begin as igneous rock, formed after magma cools, and then be transformed into sedimentary rock when compaction and cementation is present. A sedimentary rock can be transformed into a metamorphic rock with heat and pressure.
Chemical Composition
The chemical composition of minerals and rocks also differ. Few chemical elements are involved in the formation of minerals. This small group of common elements, which are found in abundance close to the Earth's crust, include oxygen, silica and iron. Rocks are generally comprised of many more chemical constituents than minerals because they are exposed to numerous elements that are only found deep within the Earth.
Classification
In most cases, minerals are classified by type in a laboratory by tediously viewing their crystalline lattice network under a microscope. For easier identification, minerals are also classified on a hardness scale. Minerals such as talc and gypsum are very soft; topaz and diamond are extremely hard. In contrast, rocks can usually be classified by a layperson with the naked eye. Sometimes, however, chemical and other specific laboratory tests are done on rocks to classify them further.
Special Properties
Chemical tests can show special properties that are specific to both rocks and minerals. Certain types of calcite exhibit an effervescent characteristic when exposed to hydrochloric acid, for example. Magnetism, another property tested in labs, is a special property found in minerals as well as rocks. These types of special properties help researchers to define the chemical constituents of both rocks and minerals even further.