Tellurium
Tellurium is a very rare mineral. There is 14 times more silver in the earth than there is tellurium. Tellurium is sometimes found as an element, but most of the times is found as tellurides. Gold tellurides (Calaverite) are found in the earth. They are valuable ores of both tellurium and gold. This gold ore was not recognized as gold during one gold rush and was used as a filler. It was then discovered that it was gold telluride, making another gold rush. Telluride cannot replace sulfide in elements like selenide does.