What is the date of discovery of platinum?

Platinum was known to at least some pre-Columbian civilizations in the New World. The earliest European reference to it is from 1557. It was first isolated in pure form (previous examples had effectively been naturally-occuring alloys) in 1786 by Chabaneau.

It turns out that it's extremely difficult to separate platinum from certain other metals (called the platinum group) using traditional metallurgical techniques.