Iron ore is the raw material from which iron is extracted.
Iron ore is a general name for a variety of rocks which contain iron, the most common being haematite. Haematite is found in geological formations known as "banded iron layers".
It is not exactly known how these layers were formed. The conventional concept is that they were formed in sea water as the result of oxygen released by photosynthetic cyanobacteria (bluegreen algae), combining with dissolved iron in Earth's oceans to form insoluble iron oxides. These chemicals then precipitated out, forming sedimentary layers at the bottom of the ocean. Plate tectonics subsequently pushed these ocean floors upwards until they were above sea level.