Ironstone is a fine-grained, heavy and compact sedimentary rock. Its main components are the carbonate or oxide of iron, clay and/or sand. It can be thought as a concretionary form of siderite. Ironstone also contains clay, and sometimes calcite and quartz.

Appearance

---

Freshly cleaved ironstone is usually gray. The brown external appearance is due to oxidation of its surface. Ironstone, being a sedimentary rock is not always homogeneous, and can be found in a red and black banded form called tiger iron, sometimes used for jewelry purposes.

Sometimes ironstone hosts concretions or opal gems.

Occurrence

---

Ironstone mostly occurs in the form of nodules. It is easy to find ironstone overlying coal measures.

Ironstone as a source of iron

---

Ironstone is a bountiful and widespread source of iron (Fe), although it only contains <50% iron, far less than the other main source of iron, hematite. Most of British iron originates from ironstone, but it is rarely used for this purpose elsewhere.

See also

---

• Iron ore
• Banded iron formation

Minerals

IJzerzandsteen