A pseudomorph is a mineral that has taken on the appearance of another mineral through a process called pseudomorphism. In this process, the original mineral's crystal structure is replaced by another mineral's structure while maintaining the outward shape of the original. This can occur through chemical reactions, hydration, oxidation, or replacement over time. Pseudomorphs can occur in various forms, such as when limonite replaces pyrite, or when quartz takes on the shape of other minerals like ammonites. Pseudomorphs are often prized by collectors for their unique and unusual appearance.