In addition to gold and silver, platinum metals such as mercury and rhenium are also counted among the classical precious metals. At room temperature in air, these elements either do not corrode at all or do so extremely slowly and to a very slight degree. Although the quantities of gold and silver in the Earth's crust are small, their existence has been known since ancient times. This is due to their precious character, their purity and the fact that they can be extracted from ore at a relatively low temperature. Since antiquity these metals have been used to produce jewelry and coins.
Contrary to the name rare earths, elements of this group are found in the Earth's crust even more frequently than some of some precious metals. These elements are termed "rare earths" because they were first discovered in rare minerals and were isolated from them in the form of oxides. The term "earth" is an old chemical term for oxides, i.e. compounds containing oxygen. The chemically-similar elements of the third group of the periodic system (with the exception of actinium) and lanthanides are considered rare earths.