Tellurium, teh LUR ee uhm, is a rare semimetallic chemical element. Pure tellurium often appears as shiny, silvery-white crystals. In nature, the element most often occurs in combination with such metals as copper, gold, lead, mercury, and silver. Pure tellurium is usually obtained as a by-product of copper refining. Tellurium is used in the production of iron, steel, and certain alloys (combinations of two or more metals). Manufacturers use tellurium in the making of semiconductors and blasting caps. Tellurium helps cure (harden) rubber and catalyzes (stimulates) chemical reactions in petroleum refining. It is also used to color glass and ceramics.