The metals are one of the types of heraldic tinctures (along with colours and furs). A metal is either or (gold or yellow) or argent (silver or white).
The middle ages made extensive use of metals for all aspects of life. Metal, however, was hard to mine (and therefore expensive) and difficult to work, especially very useful metals like iron and its derivative, steel. Common people would have had little access to rare metals, and even nobility would have felt the pinch when buying large amounts of metal, such as a suit of armour or tableware.
Metals like bronze, iron and steel for were used for weapons and armour; iron, bronze, lead, pewter and brass for tools and other items; copper, silver and gold for money and jewelry.
In ancient times only seven metals were known: gold, silver, copper, iron, quicksilver (mercury), lead, and tin (white lead).