The phrase 'diamond in the ROUGH' is a metaphor used to describe someone or something that has potential but is not yet fully developed. The phrase is often used to refer to people who have great potential but are not yet recognized for it. The crossword solutions for this phrase are ROUGH (5 letters), NEIL (4 letters), and BASE (4 letters). ROUGH refers to the idea of something being unfinished or unpolished, while NEIL and BASE refer to the diamond itself, which is a precious stone composed of carbon atoms arranged in a crystalline structure.