Marcasite is an Iron Sulphide mineral that bears many similarities to pyrite; they are often confused with each other. Indeed, in many cases jewelry that is advertised as being composed of marcasite is actually made of pyrite. One method of distinguishing between them is by smell — marcasite gives off a sulphurous odor. Specimens that are left in damp environments tend to form a green oxidation fur, and eventually degrade into and unrecognizable powder. Examples that are stored in dry atmospheres can last for a long time, however, and are therefore popular with collectors.