Refractive index: 2.02
Critical angle: 30°
Specific gravity: 5.6
Cleavage: Perfect in 1 direction.
Fracture: Subconchoidal, brittle
Dispersion: Moderately high to strong.
Heat sensitivity: Low
Birefringence (double refraction): Medium (.016)
Crystal structure: Hexagonal
Cutting: Zincite is very brittle, so be careful not to shock the stone while cutting and use fine laps for roughing in.
Polishing: Alumina or cerium on tin or tin/lead. Cerium or chrome oxide Ultralaps. Perfectly polished surfaces are nearly impossible to obtain.
Comments: Although zincite has low heat sensitivity, it absorbs and transfers heat very well. Therefore, one must be careful to keep heat from traveling through the stone to the original wax bond when transferring the stone. Zincite occurs naturally in one location: Franklin, New Jersey. Zincite that is currently on the market occurred “naturally” in the vent pipes of zinc smelters in Poland. This source of “man-made naturally occurring” zincite is now nearly “played out.” Zincite can be problematic to cut and polish, but finished stones are stunning. Cut stones slowly lose their luster with exposure to air. Careful cleaning with a soft cloth will restore the original luster.