It is estimated that between one-half to two-thirds of the value of all U.S. currency in circulation is outside of the U.S. In 1934, the $100,000 Gold Certificate became the highest denomination ever issued. It was never intended for public use. Instead, it was meant solely for official transactions between Federal Reserve Banks.
Enriched uranium. Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight. 235U is the only nuclide existing in nature that is fissile with thermal neutrons.
Uranium is a naturally occurring element that has the highest atomic weight (~238 g/mole) and is slightly radioactive. It can be found in minute quantities in most rocks, soils and waters (normally < 5 ppm), but the real challenge is to find it in high enough concentrations to make it economically feasible to mine.