The stone contains the same text in two Egyptian languages (hieroglyphic and demotic) and in classic Greek. It was used as the key to deciphering hieroglyphics by Jean-François Champollion in 1822.
The Rosetta stone was found by the French at Rosetta in Egypt.
And here is the reason I am interested in the Rosetta stone…
The term Rosetta stone is used idiomatically to refer to the key to solving a decryption or translation process. Science fiction stories often consider mathematics and science to be the Rosetta stone that will enable humans to communicate with extra terrestrial intelligences. This is because mathematics and science are intimately connected to the cosmological principle. The same rules of mathematics and science apply all over the universe and consequently would be understood by sufficiently sentient beings anywhere.