Uraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO₂, but due to oxidation the mineral typically contains variable proportions of U₃O₈. Additionally, due to radioactive decay, the ore also contains oxides of lead and trace amounts of helium. It may also contain thorium, and rare earth elements. It used to be known as pitchblende (from pitch, because of its black color, and blende, a term used by German miners to denote minerals whose density suggested metal content, but whose exploitation, at the time they were named, was either unknown, impossible or not economically feasible). The mineral has been known at least since the 15th century from silver mines in the Ore Mountains, on the German/Czech border. The type locality is the historic mining and spa town known as Joachimsthal, the modern day Jáchymov, on the Czech side of the mountains, where F.E. Brückmann described the mineral in 1772. Pitchblende from the Johanngeorgenstadt deposit in Germany was used by M. Klaproth in 1789 to discover the element uranium.