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Sedimentary
Ironstone is a sedimentary rock, which means that it's formed over time through sediment accumulation. More specifically, ironstone is a form of chemical sedimentary rock, as its formation is dependent on dissolved sediments being deposited in a solid form from a solution. In the case of ironstone, iron and oxygen mix together in a solution, and are deposited as sediment.
Dense, Heavy, and Compact
The denseness and heaviness of ironstone is due to the high levels of iron compounds found within the stone. The periodic table of elements gives a more specific indication as to the weight and density of iron. Fe (the atomic symbol for iron) has an atomic weight of 55.845, and a density of 7.874 grams per cubic centimeter, much higher than a number of other elements. The strength and durability of ironstone led to its brief popularity as a substitute for porcelain tableware in 19th century England.
Oolitic
The majority of ironstones containing iron oxides are oolitic, meaning that they are made up of masses of rounded grains (ooliths), which are built up in concentric layers. Most often, the concentric circle layers of an oolith have a diameter between 0.25 mm and 2 mm. Rocks comprised of ooliths that are larger than 2 mm in diameter are rare, and are referred to as pisolites.