With this polished stone, using an action you can cast a "Wish spell". Once the wish spell is used the stone becomes dormant and its value is a standard value. Usually, the stones are diamonds but on some occasions the stone can be a random stone of value. Emerald, Ruby or Sapphire... etc.

A Wish Stone is created by a memory from the material world where someone cast a Wish spell and the memory of it goes to the Feywild and becomes a Wish Stone. It becomes buried in Feywild and it can be mined and creatures of the Feywild will use them as currency instead of gold or silver. 

Notes: Bonus: Ability Checks, Bonus: Saving Throws, Bonus: Initiative, Buff