"If you have faith as a grain of mustard seed...nothing shall be impossible unto you. Matthew 17.20. A wonderful old Victorian belief held that the mustard seed symbolized faith. For this reason, the small golden seed was encased in glass and worn as an amulet."
"In the gospels, the mustard seed is used as a metaphor for the kingdom of heaven. Mustard Seeds are very small but they grow into very large plants that birds build nests in and call their home."
Pretty cool! A quick eBay search brought up a lot of results, in a variety of styles.