Hardystonite is a rare purple fluorescent silicate mineral from the famous Franklin, New Jersey, USA zinc deposits. Like many of the famous Franklin fluorescent minerals, it is nondescript white in daylight and is not known in well formed crystals.
See Franklin Furnace for a discussion of these deposits. Hardystonite has a chemical composition of Ca2ZnSi2O7. It was frequently found with Willemite (fluoresces green) and Clinohedrite (fluoresces orange).