Students know how to differentiate among igneous, sedimentary, and metamorphic rocks by referring to their properties and methods of formation (the rock cycle).

Since rocks are made of minerals, the California Science Framework recommends teaching about minerals (Standard 4b) before rocks (Standard 4a). The order of content in the standards does not imply any sequence in teaching the standards.

The terms igneous, metamorphic, and sedimentary indicate the processes that formed the particular rocks. Since any given rock can be made from a wide variety of minerals, there are huge variations within each of these categories. An igneous rock composed mainly of minerals X, Y, and Z may have a much greater resemblance to a metamorphic rock made of minerals X, Y and D than to another igneous rock composed mostly of minerals J, K and L. Metamorphic rocks will differ based on how much temperature and pressure formed them. Igneous rocks will differ depending on whether they solidified above or below the ground.

The net result of these and other variations is that there are no simple rules that enable one to immediately identify a particular rock as being igneous, metamorphic, or sedimentary. The California Science Framework (page 63) provides very general guidance that is summarized in the Table below, but also clearly warns that there are many exceptions to these generalizations.

Very General Descriptions of Rock Types by Rock Properties There Are Many Exceptions to These Generalizations
TYPE OF ROCK	HARDNESS	LAYERING	OTHER
Igneous	Hard	No	Interlocking crystalline textures
Sedimentary	Soft	Yes	Fragmentary textures; look like broken grains of rock cemented together
Metamorphic	Hard	Minerals lined up or arranged in uneven layers