Franklinite

Franklinite is an oxide mineral of the spinel group, with the formula Zn²⁺Fe³⁺₂O₄. It was first described in 1819, from samples from Franklin Furnace (now Franklin Borough) New Jersey. Charles Palache wrote: “The true character of the mineral was first determined by the French chemist Berthier, who named it franklinite ‘in order to remind us that that it was first found in a place to which the Americans have given the name of a great man.’”

Though Franklin is the type locality, if you count Franklin and Sterling Hill, franklinite has been found in at least 30 localities worldwide. Nowhere else is franklinite so abundant, or found in large crystals (most commonly octahedral). Franklinite is a black non-fluorescent mineral, often associated with brilliantly fluorescent calcite and willemite, and also occurs with red or orange zincite, providing very colorful specimens. Franklinite is sometimes magnetic, but is a distinct mineral species from magnetite.