I've selected "quartz" as a medium-to-hard difficulty word. It's 6 letters long and contains the uncommon letter 'q' paired with 'u', which follows a predictable pattern, but also has 'z' which is less common. The word contains a challenging consonant cluster ('rtz') that might be difficult to guess. It's a common mineral/material that most people know, but not a word used in everyday conversation. It follows all the rules: between 4-10 letters, uses only a-z, is a real English word (not a proper noun or acronym), and has no spaces or special characters.