What is the significance of the salt spoon?

In Greta's time, she finds a salt spoon while exploring one of the cellar holes in Blue Cove. Later, in one of her trips to the village of Blue Cove in the fog, Greta is with the Morrills when Retha's father gives the salt spoon to Retha. It is among some other items that he has found on a wrecked ship.