Egyptian tablet vital in the decoding of hieroglyphics (7,5)

(Other definitions for rosetta stone that I've seen before include "Inscribed slab found 1799, making it possible to decipher Egyptian hieroglyphics" , "Tablet that allowed decipherment of hieroglyphs" , "Tablet that was the key to deciphering Egyptian hieroglyphs" , "clue to civilisation" , "Champollion's key to Egyptian hieroglyphs" .)