The Goron Amber is an obtainable and recurring small treasure in both Phantom Hourglass, and Spirit Tracks. They can be sold for a significant amount of Rupees, or be used for the customization of certain objects.
Goron Ambers are scattered randomly 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 depends on the uniqueness of Link's saved file. In addition, Link can sell Goron Ambers to the Ho Ho Tribe, but only on certain days.
Just like Pearl Necklaces, Goron Ambers can be found randomly throughout New Hyrule. Goron Ambers in Spirit Tracks act almost identically to those in Phantom Hourglass. They can be sold for 500 Rupees at the Trading Post, or be used to customize Train Parts for the Spirit Train.