Hardystonite (Ca₂ZnSi₂O₇) is a calcium-zinc silicate found almost exclusively at the Franklin and Sterling Hill zinc deposits in Sussex County, New Jersey. It is famous for its violet-purple shortwave UV fluorescence — one of the most striking fluorescent mineral colors known. Discovered in 1899 and named after geologist Josiah D. Hardy.
Key Facts
- Mohs hardness 3–4.
- Tetragonal; massive habit dominates, rare crystals.
- Diagnostic intense violet-purple shortwave UV fluorescence.
- Forms in Franklin-Sterling Hill type Zn-Mn-Fe metamorphosed ore (only known major locality).
- Common in the Franklin fluorescent suite alongside willemite, calcite, esperite.
Notable Localities
Franklin and Sterling Hill (Sussex County, New Jersey, USA) are the only major source — these UV-fluorescent specimens are among the most-photographed in mineralogy.
Found at these Localities
- Franklin and Sterling Hill (富兰克林矿(新泽西))
