10 Dollars (Library of Congress; Bimetallic)

The 2000 Library of Congress $10 — the only coin the U.S. Mint ever made from two precious metals at once

In 2000 the United States Mint did something it had never done before and has never done since: it locked a disc of platinum inside a ring of gold and struck them together as a single coin. It honored the Library of Congress turning 200. It nearly didn't work — and that's a large part of why collectors chase it.