Dark Pearl Loop
Dark Pearl Loops are an obtainable and recurring small treasure in both Phantom Hourglass, and Spirit Tracks. They can either be sold for a decent amount of Rupees, or be used to customize certain Train Parts. They bear a similar resemblance to the Pearl Necklace.
Like several other items in this game, Dark Pearl Loops are randomly scattered across the World of the Ocean King. Link can sell this small treasure to the Treasure Teller on Mercay Island. The amount of rupees he receives varies on the uniqueness of Link's saved file. Otherwise, he can sell it to the Ho Ho Tribe, but only on certain days.
Dark Pearl Loops act almost identically in Spirit Tracks, to those in Phantom Hourglass. They are randomly scattered across the land of New Hyrule. They can be sold for 150 rupees at the Trading Post, or used for the customization of Train Parts for the Spirit Train.