Iron is by mass the most common element on Earth, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust. Pure iron is relatively soft, but is unobtainable by smelting. The material is significantly hardened and strengthened by impurities, in particular carbon, from the smelting process. A certain proportion of carbon (between 0.002% and 2.1%) produces steel, which may be up to 1000 times harder than pure iron.
Natural stones (stones that can be used in jewelry after cutting and polishing) are well-known for their vibrant colors, but are not one of the four precious stones (diamond, emerald, ruby, sapphire).