Ironstone

A sedimentary rock formed either by the direct deposition of iron rich material or through chemical replacement processes, containing substantial amounts of iron bearing minerals that can include oxides such as limonite, hematite and magnetite, carbonates such as siderite, silicates such as chamosite, or combinations of these. A freshly broken surface typically appears grey, though weathering and oxidation give the rock its familiar brown exterior, and its composition is not always uniform, occasionally showing red and black banding known as tiger iron. Ironstone was historically a major source of iron in Britain, though it is now rarely mined for that purpose because the quantity available is small relative to demand, with most modern iron smelting instead relying on banded iron formations, which supply nearly two thirds of the world's iron ore. Beyond iron extraction, ironstone has also been used as a building material for walls and structures, and despite the similar name, the nineteenth century ceramic ware known as ironstone china contains no actual ironstone but is valued for being resistant to chipping.