A common misperception among those new to metal detecting is that metal detectors can identify one metal from another. How much we wish that were true. The reality is that for all practical purposes the common metal detector target id scale is based on a combination of the conductive or ferrous properties of the item multiplied by the size and shape of the item.
There are two common terms in use for this scale. The Target ID or TID scale is the most generic. White's also popularized the use