Why is it silver that kills a werewolf?

Aside from just the idea that it has to be "something" (maybe lead, steel, whatever), what is it about silver that makes it so special that it can kill a werewolf where other bullets or a stake to the heart can't?

Answer

That connection, which is now an un-avoidable cliché, was made up. However, that belief is so entrenched in the mythology's conventions that it has now become an integral part of it.

"Most modern fiction describes werewolves as vulnerable to silver weapons and highly resistant to other injuries. This feature does not appear in stories about werewolves before the 20th century (the claim that the Beast of Gévaudan, an 18th-century wolf or wolf-like creature, was shot by a silver bullet appears to have been introduced by novelists retelling the story from 1935 onwards and not in earlier versions).[37]" http://en.wikipedia.org/wiki/Werewolf

This silver bullet needed for killing makes sense under the Magical Law of Sympathy http://en.wikipedia.org/wiki/, Sympathetic_magic, or associative magic, according to the Law of Contagion

“The law of contagion is a folk belief axiom found in magical thinking which suggests that, once two people or objects have been in contact, a magical link persists between them « http://en.wikipedia.org/wiki/Law_of_contagion:

On the surface it makes sense, however silver (Argentium) is only one of the metals to be associated with the moon, along with any grey or silvery ones like lead, mercury or quicksilver, or Selenium. Its main association with the moon seems to have been because it was one of the only 7 metals found in antiquity and they made them correspond to the planets:

"The ancients believed in a connection between these seven metals and the seven Classical planets. "http://en.wikipedia.org/wiki/Metals_of_antiquity

Besides he proper moon goddess is Selene, not Artemis

“Both Selene and Artemis were also associated with Hecate, and all three were regarded as lunar goddesses, although only Selene was regarded as the personification of the moon itself. Her Roman equivalent is Luna.[2] » http://en.wikipedia.org/wiki/Selene

Note that Luna or Lune is the name of the moon itself is several Latin languages, hence loony, lunatic, lunar, also:

“Greek selene (Lesbian selanna) is from selas "light, brightness (of heavenly bodies)” http://www.etymonline.com/index.php?term=moon&allowed_in_frame=0