Psilomelane is a term used to describe a group of hard, black, hydrous manganese oxide minerals, rather than a single mineral species. It's often found in massive, botryoidal, or stalactitic forms. While historically psilomelane was considered a mineral, it's now understood to be a mixture, with romanechite being a common constituent. Psilomelane is an important ore of manganese and can be found in various geological settings.