Fe304 = FeO * Fe2O3, which are iron oxide ions with valence of +2 and +3 respectively. The color is black, not magnetite.
Fe2O3 is iron oxide, in which the iron ion is +3 and the color is red, which is hematite.
How did Feo come into being?
It can be prepared by heating ferrous oxalate, which is decomposed by heating to generate ferrous oxide, carbon monoxide and carbon dioxide.
FeC2O4 → FeO + CO↑ + CO2↑