I've selected "quartz" as a medium-to-hard difficulty word. It's 6 letters long, which is within the required range. The word contains uncommon letters like 'q' and 'z' which makes it challenging, but it's still a recognizable word that refers to a common mineral. It follows all the rules: it's between 4-10 letters, uses only standard English letters, is a real English word (not a proper noun or acronym), and has no spaces or special characters.