Ironstone is an ancient medium- to fine-grained sedimentary rock containing iron-rich minerals such as hematite and siderite; magnetite and pyrite may also be present. Ironstones were formed two to three billion years ago in the Precambrian era. In banded ironstone, the basic rock is chert interspersed with layers of ferruginous minerals. Oolitic limestone, formed in the bottom of sediments at the bottom of ancient seas, consists of small rounded grains (ooliths) in which ferruginous minerals have replaced lime-rich ones. In both types, the iron-containing minerals result in a deep red color.