Latinum

In the universe of Star Trek, Latinum is a major currency. Latinum's dominance as a form of currency is a result of its popularity with the Ferengi. Latinum is a liquid with the unique property that its molecular structure cannot be replicated or synthesized, providing the condition of shortage necessary for a substance to be used as currency. As it is difficult to properly measure liquid for currency transations, premeasured amounts of latinum are inserted into hollow cores of gold bullion of various sizes, leading to the standard units of gold-pressed latinum: slip, strip, bar, and brick.

For an exposition on the future of money and how latinum might come to be, see "Proposal for an Ideal Nano-Specie: Gold-Pressed Latinum" by Robert Freitas [1].