I've selected "quartz" as a medium difficulty word. At 6 letters, it fits within the length requirements. It's a real English word (a common mineral) but contains the uncommon letters 'q' and 'z', which makes it challenging to guess. It's not a proper noun, contains no special characters or spaces, and uses only standard English letters. The uncommon letter placement creates a good balance - recognizable enough but still providing a fair challenge for the opponent.