For the discovery of what phenomenon was Einstein awarded the Nobel Prize for 1921?

Einstein is known the world over for his revolutionary work in propounding the Special and General Theories of Relativity (in 1905 and 1916). However, he was awarded the Nobel Prize for his work on Photoelectric Effect which was easier to prove experimentally. That does not in any way detract from the fact the Photoelectric Effect was an equally (if not more) revolutionary discovery since it forms the basis of Quantum Physics.

During those days the Nobel committee was partial towards theories which could be proven through experiments and neither Special or General Theory of Relativity fulfilled that criterion. In fact Einstein's prediction of the Gravitational Wave (part of the General Theory of Relativity) could be experimentally proved only in 2016.