Giving a jewel means giving a unique object, representing eternity and beauty. Precious gifts are inevitably linked to the theme of romance: it is romantic to give something that your partner desires, and it is wonderful the feeling that you feel when the other discards the gift and does it with happiness. A jewel is something that is then worn, carried on, in contact with the skin.
We give you some happiness to keep with you at all times.
This is behind a jewel, not just an ornamental object.