Ironstone

Lithified iron-rich sediments.
Also known as: iron formation.

Any rock containing a substantial proportion of an iron compound, or any iron ore from which the metal may be smelted commercially; specifically, an iron-rich sedimentary rock, either deposited directly as a ferruginous sediment or resulting from chemical replacement. The term is customarily applied to a hard, coarsely banded or non-banded, and non-cherty sedimentary rock of post-Precambrian age, in contrast with banded iron formation.
The iron minerals may be oxides (limonite, hematite, magnetite), carbonate (siderite), or silicate (chamosite); most ironstones containing iron oxides or chamosite are oolitic.

Clay rich varieties are called clay ironstone.

A chemical sedimentary rock, typically thin-bedded and/or finely laminated, containing at least 15% iron of sedimentary origin, and commonly, but not necessarily, containing layers of chert. Various primary facies (usually not weathered) of iron formation are distinguished on the basis of whether the iron occurs predominantly as oxide, silicate, carbonate, or sulfide. Most iron formation is of Precambrian age. In mining usage, the term refers to a low-grade sedimentary iron ore with the iron mineral(s) segregated in bands or sheets irregularly mingled with chert or fine-grained quartz. Essentially synonymous terms are itabirite; banded hematite quartzite; taconite; quartz-banded ore; banded iron formation; calico rock; jasper bar.

Compare with: jaspilite.

Ref: AGI