Franklinite is an important zinc ore mineral having the formula (Zn,Fe,Mn)(Fe,Mn)2O4 - zinc iron manganese oxide. It has a metallic luster, black color, no cleavage, and is slightly magnetic. Most crystals have a truncated rounded shape. Well formed crystals are nice octahedrons (double square pyramids).
The only significant franklinite occurrence on Earth is in northern New Jersey, in the famous Mesoproterozoic-aged Franklin Marble. In that unit, the rare zinc minerals franklinite and zincite (red ZnO) are abundant and mined as a source of Zn metal.
Franklinite crystals (most are 1 to 2 mm in size) from residue of crushed zinciferous marble (zinc ore: franklinite-zincite-willemite-calcite rock). From the Mesoproterozoic-aged Franklin Marble at the Sterling Hill Mine in Franklin, northern New Jersey, USA.