Explaining the magnetism is not easy but here is a go at it. Remember, electricity produces magnetic fields just as magnetism produces electic fields. The A and B represent usually different metal ions that occupy specific sites in the crystal structure. This arrangement causes a transfer of electrons between the different irons in a structured path or vector. This electric vector generates the magnetic field.
Color is black.
Luster is metallic to dull. Crystals are opaque. Twinning of octahedrons into spinel law twins is seen occassionally. Cleavage is absent although octahedral parting can be seen on some specimens. Fracture is conchoidal.