I've chosen "quartz" as a medium-difficulty word for Hangman. It's 6 letters long (within the 4-10 range) and is a common mineral, so it's a real English word but not a proper noun. It contains the uncommon letters 'q' and 'z', which makes it challenging, but it's still a word most people would know. The word uses only standard English letters with no spaces, hyphens, or special characters.