Some metals like gold (Au), silver (Ag) and copper (Cu) are available in nature in free state (native) as they are least reactive. Other metals mostly are found in nature in the combined form due to their more reactivity. The elements or compounds of the metals which occur in nature in the earth crust are called minerals. At some places, minerals contain a very high percentage of a particular metal and the metal can be profitably extracted from it. The minerals from which the metals are extracted without economical loss are called ores.

Aluminium, for example, is the most common metal in the Earth’s crust, occurring in most of minerals. However, it isn’t economically feasible to extract it from most of these minerals. Instead, the usual ore from which it is profitable to extract it is bauxite – which contains from 50 – 70 % of aluminium oxide