What is "rhombohedral iron ore"

Hematite \Hem"a*tite\, n. [L. haematites, Gr. ? bloodlike, fr. a"i^ma, a"i`matos, blood.] (Min.) An important ore of iron, the sesquioxide, so called because of the red color of the powder. It occurs in splendent rhombohedral crystals, and in massive and earthy forms; -- the last called red ocher. Called also specular iron, oligist iron, rhombohedral iron ore, and bloodstone. See Brown hematite, under Brown.