Of no practical use whatsoever, and one of the rarest naturally ocurring elements, Protactinium is found in minute quantities as a by-product of uranium decay. Predicted to exist by Mendeleev the missing element  91 was actually discovered in the laboratory in 1913.  In 1961 scientists in England were able to produce  about 4 ounces of 99.9% pure protactinium, although they had to process about 60 tons of radioactive waste and spend about $500,000 to get it.

With a half life of 32,760  years, Protactinium is highly radioactive and highly toxic.  Found in only 1 part per trillion in the environment, Protactinium was discovered in the laboratory and, thankfully, there it remains solely as the subject of basic research.