I selected "quartz" as a medium difficulty word for Hangman. It's 6 letters long (between the required 4-10 letters) and is a common mineral that most people know. What makes it challenging is that it contains the uncommon letters 'q' and 'z', while also having the common vowel 'a'. This creates a good balance where the player might get a few letters early but still need to work for the solution. It's a real English word with no special characters, spaces, or hyphens, and isn't a proper noun or acronym.