Hardystonite is a rare calcium zinc silicate mineral first described from the Franklin, New Jersey, U.S. zinc deposits. Like many of the famous Franklin minerals, hardystonite responds to short wave ultraviolet (254 nm wavelength) light, emitting a fluorescence from dark purple to bright violet blue. In daylight, it is white to gray to light pink in color, sometimes with a vitreous or greasy luster. It is very rarely found as well formed crystals, and these are usually rectangular in appearance and rock-locked.
thumb|left|Hardystonite in plain light, same sample as in fluorescent light image above right
Hardystonite has a chemical composition of Ca<sub>2</sub>ZnSi<sub>2</sub>O<sub>7</sub>. It is frequently found with willemite (fluoresces green), calcite (fluoresces red), and clinohedrite (fluoresces orange). Hardystonite can be found altered to clinohedrite CaZn(SiO<sub>4</sub>)·H<sub>2</sub>O through direct hydrothermal alteration.