A noble metal is ordinarily regarded as a metallic chemical element that
is generally resistant to corrosion and is usually found in nature in its raw
form. Gold, platinum, and the other platinum group metals (ruthenium, rhodium,
palladium, osmium, iridium) are most often so classified. Silver, copper and
mercury are sometimes included as noble metals, however less often as each of
these usually occurs in nature combined with sulfur.
In more specialized fields of study and applications the number of elements
counted as noble metals can be smaller or larger. In physics, there are only
three noble metals: copper, silver and gold. In dentistry, silver is not
always counted as a noble metal since it is subject to corrosion when present
in the mouth.