Rock Topics

Rock Basics

• Rocks are natural solids composed of mineral grains, glass or a combination of these.  Most are mixtures of different kinds of minerals.

Natural means that rocks are formed through natural processes.  Human-made materials, such as concrete and slag, are not rocks.

Solid means that rocks are hard and consolidated.  Loose fragments (like sand) are not a rock.

Mineral grains means that most rocks are mixtures of one or more mineral types.

Glass includes the natural volcanic glasses, which are not true minerals because they lack crystal structure.

Granite Granite used in a Richmond building (Photograph by Parvinder Sethi)

Granite, shown here, is a type of rock.  This piece of granite is made up of the minerals feldpar (pink), biotite (black), and quartz (gray).  This example of Petersburg Granite is used as a building stone in downtown Richmond.