What are the precious things mentioned in the story? Why are they precious?

The leaden heart of the Happy Prince and the dead swallow are the two most precious things mentioned in the story.
They are precious because the Happy Prince was kind and served the poor and the needy by providing them with the last resource of his statue to bring back a little happiness in their lives. The bird was always been his faithful messenger and helped him helping the unprivileged. When God asks an angel to bring two precious things in the city, it brings him the leaden heart of the Happy Prince and the dead swallow.