Hematite, iron oxide, is one of the most common and abundant minerals on the earth's surface, and provides yellow, orange and red coloration to rocks throughout the world. It forms in many different geochemical environments, and can thus appear in many forms; The photo shows (L-R) banded hematite that precipitated from water in layers; massive hematite that occurs in veins; and oolitic hematite, consisting of 'oolites', tiny spherical particles that form in water and precipitate as aggregates.