kwarts n (plural kwartsen)
German Quarz, from Middle High German twarc, probably from a West Slavic language (compare Czech tvrdý (“hard”), Polish twardy (“hard”), Russian твёрдый (tvjórdyj, “hard”)), from Old Church Slavonic тврьдъ (tvrĭdŭ, “firm”), from Proto-Slavic *tvьrdъ.