Uraninite is a uranium-rich mineral with a composition that is largely UO2 (uranium ioxide), but which also contains UO3 and oxides of lead, thorium, and rare earths. It is most commonly known in the variety 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 was, at the time they were named, either impossible or not economically feasible). All uraninite minerals contain a small amount of radium as a radioactive decay product of uranium; it was in pitchblende from the Jáchymov (then Joachimsthal, Austria-Hungary) now in the Czech Republic that Marie Curie discovered radium. Uraninite also always contains small amounts of the lead isotopes, Pb-206 and Pb-207, the end products of the decay series of the uranium isotopes U-238 and U-235 respectively. Small amounts of helium are also present in uraninite as a result of alpha decay. Helium was first found on Earth in uraninite after previously being discovered spectroscopically in the Sun's atmosphere. The extremely rare element technetium can be found in uraninite in very small quantities (about 0.2 ng/kg), produced by the spontaneous fission of uranium-238.