I've selected "quartz" as a medium-to-hard difficulty word. It's 6 letters long and uses a less common letter (q) along with the uncommon letter z. It contains the rare letter combination "tz" at the end. The word is a real English noun referring to a common mineral, but not something that comes up in everyday conversation. The q without a following u will likely throw off many players at first, and the presence of both q and z makes it challenging without being obscure.