As you can see, it looks like a crescent moon. That's because in ancient days, people believed that the moon was made of silver. So how did the myth arise that werewolves were harmed by silver if people believed this?
A modern paper and pencil game sheds a little light on this oxymoron. The game, called Werewolf: The Forsaken.
Quoted from Wikipedia:
"...Over time Luna, the spirit of the moon, began to travel the world in a body of flesh. She had many suitors but choose only Father Wolf as a mate, due to his strength and power. With him she sired the First Pack (children part human, part wolf, and part spirit), who were the ancestors of all Uratha and assisted their father in his duties.
Over time Father Wolf grew weak and was no longer able to fulfill his duties. When his children saw his weakness, they began to rebel against their father and succeeded in killing him and taking his place. As Father Wolf died, the barrier between the worlds became so strong that traveling from one to another was only possible in special places for most beings. The pain over the death of her lover caused Mother Luna to curse all children she had ever borne, so that silver (her sacred metal) is able to kill them..."