I saw this on Offbeat Bride today and really want to do it now - I thought it was awesome and so symbolic. They have one for a unity candle, one for a gift, and one for a mixed family.
http://offbeatbride.com/2010/06/salt-ceremony-wedding-gift
~~~~~The Story of the Salt Covenant~~~~~~
The Salt Covenant
During ancient times, agreements and promises were sealed by a salt covenant. Each person would take a pinch of salt from their pouch and place it in the pouch of the other. This agreement could not be broken unless an individual could retrieve their own grains of salt. But this, of course, is impossible.
This raku-fired salt egg has been especially made for you to create a salt covenant with your spouse in the beginning of your new life together.
Both spouses should take a bag of salt and combine the the two bags inside the egg.This a bond that can not be broken... as is a symbol of an unbreakable promise of love.