👉 Tourmaline is a mineral that is formed in the Earth's crust from the reaction between iron and copper. It is a greenish-brown to brown-colored mineral, which occurs in various forms, including polished tourmalines, iridescent tourmalines, and tourmaline crystals. Tours are made by the process of chemical precipitation, where an aqueous solution of iron (III) chloride dissolves into water, leaving behind a precipitate. The resulting tourmaline crystals