On a recent trip out to the desert near Boron, a group of us went out collecting rocks.
The area we were at was know for its chalcedony (which is pronounced something like kal-sed-a-nee). This is actually the name of a large family of rocks and minerals including agates and jasper. But the form that is generally just called chalcedony is a bluish-grey quartzy rock.
The crystals in chalcedony are typically very small. The composition is mainly silica (silicon dioxide), while the colors come from other chemicals in the stone.