Noble metal
A noble metal is a metal that is resistant to both corrosion and oxidation when exposed to air.  Metals such as gold, silver, platinum, iridium, rhodium, palladium and rhenium are examples.  Sometimes, mercury and copper can be regarded as noble metals.  Some metals like titanium are not, even though they are resistant to corrosion.  Noble metals are valuable, because they are not commonly found in the Earth's crust.  They have many uses in everyday things including chemical engineering, electronics, and jewelry.