* Mineral: A mineral is a naturally occurring, solid, inorganic substance with a defined chemical composition and a specific crystalline structure. While alum crystals have a defined chemical composition (potassium aluminum sulfate), they are not naturally occurring. They are produced through human processes.

* Rock: A rock is a naturally occurring solid aggregate of one or more minerals. Alum crystals are not aggregates of minerals; they are a single substance.

Alum crystals are a chemical compound, specifically a double sulfate salt, often used in various applications like dyeing, water purification, and baking. They are formed through human intervention and are not found in nature.