Franklinite

Franklinite is a black, weakly magnetic oxide of the spinel group — a zinc-manganese-iron oxide — and one of the principal ores of the famous Franklin zinc deposit in New Jersey, after which it is named.

Identifying franklinite

Franklinite forms black octahedral crystals and rounded grains with a metallic to submetallic lustre and a dark red-brown streak. It is weakly magnetic, which helps distinguish it from look-alikes, and is almost always found with the other Franklin ore minerals.

The Franklin connection

The Franklin and Sterling Hill deposits in New Jersey are essentially the world's only significant source of crystallised franklinite. There it occurs with willemite, zincite and calcite in the celebrated fluorescent ore — sharp black franklinite octahedra set in calcite that glows red under ultraviolet light are a signature display.

For collectors

Well-formed black octahedra in red-fluorescing calcite, and the classic Franklin three-mineral assemblages, are the most prized.