The terms Acid and Basic do not refer to the pH of the rock: it is based on the percentage of silica in the rock.

The rocks in the Granite 'family' contain more than 10 percent Quartz. In the two Intermediate families Orthoclase is dominant in the Syenite family and Plagioclase is dominant in the Diorite family. In the Basic rocks Plagioclase and 'Augite' (Pyroxene Group minerals) are dominant, and dark minerals dominate the Ultrabasic rocks.

Generally the Acid rocks are lighter in colour than the more basic ones.