A King’s Zinc Mineral and Fluorescing Beauty

STORY BY BOB JONES

This specimen of willemite, with the fluorescent view as well, was mined at the Garpenberg Södra Mine, Dalarna County, Sweden.

HANNES GROBE, WIKIMEDIA COMMONS

It is no accident that Franklin, New Jersey, is the “Fluorescent Mineral Capital of the World.” After all, the twin complex zinc-iron deposits of Franklin-Sterling Hill have managed to produce something over 70 minerals that respond to some form of ultraviolet excitation.

But you have to wonder if Franklin, New Jersey town would be so honored to be this specific “capital” if willemite did not respond to ultraviolet excitation. But this reaction is just one of willemite’s essential properties.

The fluorescence of this zinc silicate is so dependable at Franklin that ultraviolet light is present on the picking table to beneficiate the ore as it comes to grass. Willemite will sometimes phosphoresce for over a dozen hours. Another odd property of willemite is visible when you strike a massive chunk with a hammer. It will briefly phosphoresce. These properties are fascinating, but willemite’s importance in New Jersey is its zinc content. The two New Jersey deposits of Franklin-Sterling Hill were the only major producers of that useful metal.

From the famous Franklin Ore Body, in Sussex County, New Jersey, is the material of willemite, calcite and franklinite, that this sphere is carved. The willemite appears as tones of brown under normal lighting, and electric green when it fluoresces.

HERITAGE AUCTIONS, HA.COM

WILLEMITE AND FRANKLIN-STERLING HILL CONNECTION

Mention willemite to any collector, and, likely, they’ll instantly think of Franklin-Sterling Hill, New Jersey. That’s because these are the only deposits where it is the primary zinc ore. Another major source of willemite was Tsumeb, Namibia, but that was never as important as a zinc source as New Jersey. Yet, it yielded excellent zinc minerals like smith-sonite of fine quality. To everyone’s surprise, Tsumeb willemite, however, does not fluoresce.