What does the mustard seed symbolize in the christian faith?

I have seen necklaces and such with a mustard seed in them at christian stores. Why is this? What does the mustard seed symbolize?

In the Bible: Has any Christian a Faith as Small as a Mustard Seed?

The mustard is mentioned five times in the Bible, all of them are in the New Testament being not mentioned at all in the Old Testament. 
The mustard is mentioned two times in the Quran.
Jesus rebuked the demon, and it came out of the boy, and he was healed from that moment.
Then the [...]