Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement, that contains a substantial proportion of an iron ore compound from which iron (Fe) can be smelted commercially.

Palaeontological and mineralogical evidence indicates that these ironstones were formed during freshwater inundation.

Oolitic Ironstones The deposits were formed in shallow, near-shore marine environments and are most extensively developed in England, the Lorraine area of France, Belgium, and Luxembourg. In North America oolitic iron deposits contain ooliths of hematite, siderite, and chamosite and are called Clinton-type deposits.

Ironstone is any chemical sedimentary rock with >15% Fe. An iron formation is a stratigraphic unit which is composed largely of ironstone. Most genetic modellers attribute cherty laminated iron formations to hydrothermal convection and noncherty oolitic iron formations to surficial weathering.

Oolitic limestone is made up of small spheres called ooiliths that are stuck together by lime mud. They form when calcium carbonate is deposited on the surface of sand grains rolled (by waves) around on a shallow sea floor.

How to Identify Ironstone. If you’re lucky, it will be labeled as ironstone in the hallmark, usually stamped on the bottom of a piece. It might also say “stoneware”. Many pieces, though, are not marked specifically as ironstone or they aren’t marked at all.

Composition. Ooids are most commonly composed of calcium carbonate (calcite or aragonite), but can be composed of phosphate, clays, chert, dolomite or iron minerals, including hematite. Dolomitic and chert ooids are most likely the result of the replacement of the original texture in limestone.

the Great Salt Lake
Oolitic sand is an unusual sediment that is found in and around the Great Salt Lake. Instead of forming from grains of mineral fragments washed down from higher ground, this sand formed within the Great Salt Lake. It is composed of tiny, lightbrown, rounded oolites.

A piece of ironstone will always feel heavier than it looks. It has a wonderful luster about it as well, that can be easily recognized if you know what you’re looking for. If the piece has a handle, hold it by the handle and flick the body of the piece. It will make a lovely “ring” if it is free of chips or cracks.

Most, but not all, ironstone is marked with a stamp on the bottom that is printed, impressed or both. Color. Early English pieces made for export will have a blue or gray tint. Pieces that remained in England are creamier white, as are American ironstone pieces.

Oolites form today in warm, supersaturated, shallow, highly aggitated marine water. They are commonly associated with zones of high tidal activity in a subtidal or lower intertidal environment. The mechanism of formation is to begin with a seed of some sort, perhaps a shell fragment.

The Cleveland Ironstone Formation is a sequence of marine ironstone seams interbedded with shale and siltstone units which collectively form a part of the Lower Jurassic System of rocks underlying Cleveland in North Yorkshire.

Exploitation of the ironstone seams became a major driving force behind the industrialisation of the Teesside district during the mid- to late-1800s. Based on the stratigraphy of the formation, the Cleveland Ironstone was found to belong within the Upper Pliensbachian (Domerian) universal stage.

On average the Cleveland Ironstone Formation comprises around 70% shale and 30% ironstone though the latter occurs in the form of the six named seams of variable thickness. The ironstone is classed as being low-grade with an iron-content of up to 33%, and is deemed economically viable only above ~27%.

Idealized stratigraphical column for blackband ironstones showing relative water depths of sedimentation, not to scale. Ironstones and iron formations are iron-rich chemical precipitates from the ocean that were deposited episodically throughout Earth’s history, displaying different sedimentological and geochemical characteristics through time.