Royal mine is a term that refers to a type of mine that has a special connection or attachment to a royal family. In other words, it's a mine with royal patronage or support. However, if you're looking for synonyms for the phrase "royal mine," you can go for alternatives such as "monarchial mine," "majestic mine," "noble mine," or "imperial mine." These synonyms suggest the same association with royalty, grandeur, or high status. They are perfect for use in various contexts, including literature, history, or even business communications, where operators want to convey their mine's importance, distinction, or prestige.