Two of the rarest natural elements on earth are Astatine and Francium. Astatine is so rare, that it is practically unstudied. It has a name and a place on the periodic table, but nothing else. Francium is so rare that it is thought that at any given moment there may be fewer than twenty francium atoms on earth.
Astatine is the only element whose discovery was confirmed by a non-primate. Emilio Segre had identified Astatine in 1939 and injected it into a guinea pig to study it. Because, astatine is below iodine in the periodic table, the body thinks it to be iodine and handles it as such. So in the guinea pig, it was selectively filtered and concentrated by the rodents thyroid gland. Pretty neat trick!
I know the above elements like many other in the periodic table are quite obscure, but that does not mean obscure elements come in trace quantity. Take for example cerium, there is more of it on earth than copper. There is more neodymium and lanthanum than cobalt or nitrogen. Not surprised yet. Praseodymium, samarium, gadolinium and dysprosium are more abundant than tin.