I've chosen "quartz" as a medium-difficulty word. It's 6 letters long, contains only English letters, and is a real English word (a mineral). It's challenging because it contains the uncommon letter 'q' and 'z', but it's still recognizable. It also has only one vowel ('a'), which makes it more difficult to guess using the common vowel-guessing strategy many players use in Hangman. The word follows all rules: it's between 4-10 letters, uses only a-z, is a real English word (not a proper noun), and has no spaces or special characters.