Gold is yellow because of relativity

Without special relativity, gold's electron structure would make it white, like silver. Relativistic contraction of the inner orbitals shifts the absorption that gives gold its colour.

In heavy atoms the inner electrons move fast enough that their mass increases and their orbitals contract. In gold that contraction shifts the energy gap between the 5d band and the 6s band. The metal absorbs blue and violet and reflects the rest, which the eye calls yellow. Run the same calculation without relativity and the absorption moves into the ultraviolet, where silver keeps it, and gold comes out white.

The same physics helps explain why mercury is a liquid and why lead-acid batteries have the voltage they have. Relativity is not only a theory of spacecraft. It is in the periodic table from the sixth row down.