Fortunately for geologists, only about one hundred of these are abundant and common in the rocks of the Earth's crust. The list of common minerals in volcanic rocks is even smaller. The minerals can be grouped by color. These minerals are often referred to as mafic or ferromagnesian minerals. The absence of magnesium and iron in a mineral results in a relatively lighter color.
Geologists use the physical properties of a mineral to identify it in the field.
These properties include color, streak, luster, cleavage, specific gravity, and hardness .