Rosetta stone (n.)
A Rosetta stone is a clue or discovery that makes a difficult mystery possible to understand.
The original Rosetta Stone was found in Egypt in 1799. The same decree had been written on it in three scripts: Egyptian hieroglyphs, Demotic, and Ancient Greek. Scholars could read the Greek. For the first time, they had a way to compare a known text with writing they could not yet read.1
Thomas Young made important early advances. In 1822, the French scholar Jean-François Champollion made the decisive breakthrough and showed that hieroglyphs could record the sounds of the Egyptian language. The old writing began to speak again.1
The name outlived the stone. A Rosetta stone is now anything that provides the crucial key to understanding something that had resisted explanation.
There is a replica in the National Cryptologic Museum beside the headquarters of the NSA. It is difficult to imagine a more appropriate place for it.2
The new evidence proved to be a Rosetta stone.