The Pearl Necklace is an obtainable treasure in both Phantom Hourglass, and Spirit Tracks. They can be sold for a significant amount of Rupees, or be used to customize certain objects. They also bear a very similar resemblance to the Dark Pearl Loop.
Pearl Necklaces are scattered across the World of the Ocean King in Phantom Hourglass. Link can sell this small treasure to the Treasure Teller on Mercay Island. The number of rupees Link receives depends on the uniqueness of the players saved file. Their worth can range from a whopping 1,500 rupees, to a decent amount of 50 rupees. Pearl Necklaces can also be sold to the Ho Ho Tribe, but only on certain days.
Pearl Necklaces in Spirit Tracks act almost identically to those in Phantom Hourglass. They are scattered randomly across New Hyrule. They can be sold for 150 rupees at the Trading Post, but can also used for the customization of Train Parts.