 With an estimated average concentration of 1 part per billion (ppb), rhenium is one of the rarest elements in the Earth's crust.Rhenium has the third-highest melting point and second-highest boiling point of any stable element at 5903 K. Rhenium resembles manganese and technetium chemically and is mainly obtained as a by-product of the extraction and refinement of molybdenum and copper ores.Rhenium shows in its compounds a wide variety of oxidation states ranging from −1 to +7.