THE GEMSTONE LIBRARY
Willemite (New Jersey)
A quiet speckled sphere in daylight can reveal a very different mineral landscape under ultraviolet light.
Our March collection in ordinary light
The photograph shows polished spheres with white, pink and dark speckling, including four held in a hand above more spheres. This is an ordinary-light image, so it does not show their UV response. The collection is labeled Willemite from New Jersey. A polished sphere presents the patterns in the material across a rounded surface; where several minerals occur together, it is more accurate to describe willemite-bearing material than to assume that every visible area is pure willemite.
A zinc mineral and a historic district
Willemite is zinc silicate, Zn₂SiO₄, and was an ore mineral in the Franklin and Sterling Hill district of New Jersey. The Franklin-Ogdensburg Mineralogical Society describes it in granular aggregates, often associated with franklinite and zincite in a calcite matrix, as well as in crystals, veins and other forms. Its ordinary-light appearance is remarkably varied: green is only one of many possible colors. The pale and dark areas of a polished rock should not be assigned mineral names from a photograph alone, even when the general association is well known.
Why collectors look under UV
New Jersey willemite is especially celebrated for bright green fluorescence. The local mineralogical society reports this response in almost all willemite from Franklin and Sterling Hill under shortwave ultraviolet light. Trace manganese substituting for zinc can activate the green emission. UV provides energy to the material, which then emits visible light; the observed color is not simply the mineral reflecting a green lamp. A response can vary with composition and with the wavelength and strength of the light used. A common longwave flashlight therefore should not be assumed to reproduce every shortwave display.
Fluorescence, afterglow and mineral mixtures
Some willemite specimens continue to emit light after the UV source is removed, a phenomenon called phosphorescence. The Fluorescent Mineral Society documents an example of radiating Franklin willemite with prolonged afterglow. That is a specimen-specific observation, not a promised duration for our spheres. When willemite occurs alongside other minerals, UV can reveal a contrasting pattern: some components emit different colors and others remain dark. Comparing an ordinary-light image with a properly recorded UV image of the same piece is particularly informative. Recording the lamp wavelength helps make that comparison meaningful.
Caring for polished spheres
The mineral's published hardness is around 5.5, while a mixed rock may contain components with different properties. Keep spheres on stable padded stands, avoid knocks and protect polished surfaces from abrasive dust. A soft cloth and gentle cleaning are preferable to harsh scrubbing. The New Jersey designation describes this collection's supplied provenance; it does not identify which mine or level produced each sphere.
Sources and further reading
- Franklin-Ogdensburg Mineralogical Society · Willemite
- Fluorescent Mineral Society · Radiating willemite from Franklin
- Handbook of Mineralogy · Willemite
From our collection
