Any of a group of sedimentary rocks that consist largely or almost entirely of silicon dioxide (SiO 2 ), either as quartz or as chert , the most common siliceous rock.
It occurs in beds and in nodules. Bedded cherts may be an original organic or inorganic precipitate. Nodular cherts appear to be produced by the alteration of preexisting sedimentary rock. In this process silica distributed throughout the rock dissolves and reprecipitates to form nodules.