The coin world is abuzz over the auction of a rare silver dollar, one of the most valuable in the world and one of only 15 known to exist from a never-circulated group made for the likes of the King of Siam and the Sultan of Muscat.

The 1804 Adams-Carter silver dollar fetched more than $2 million in a private sale two years ago and is expected to top that again this week. The coin has been owned by a Boston banker, a Texas publishing mogul and by a collector who sold everything to help build a church school in Ohio.