polianite

Century Dictionary and Cyclopedia

1. n. Anhydrous manganese dioxid (MnO₂), a mineral of a light steel-gray color and hardness nearly equal to that of quartz. It crystallizes in tetragonal forms, and is isomorphous with rutile (TiO₂), cassiterite (SnO₂), and zircon (ZrO₂SiO₂). It has often been confounded with the commoner mineral pyrolusite.

Wiktionary

1. n. mineralogy manganese dioxide, occurring in hard tetragonal crystals

GNU Webster's 1913

1. n. (Min.) Manganese dioxide, occurring in tetragonal crystals nearly as hard as quartz.