Color a The overall lightness or darkness of a rock is a valuable indicator of its makeup. a Light-colored rocks are Quartz. Feldspar. Muscovite Dark-colored rocks are Biotite Amphibole PyroxeneColor ◼ The overall lightness or darkness of a rock is a valuable indicator of its makeup. ◼ Light-colored rocks are: ◼ Quartz. ◼ Feldspar. ◼ Muscovite. ◼ Dark-colored rocks are: ◼ Biotite. ◼ Amphibole. ◼ Pyroxene