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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. Historically, most of British iron originated from ironstone, but it is now rarely used for this purpose.

See also

• Iron ore
• Banded iron formation