The Rosetta Stone is one of the most valuable objects in the British Museum as it holds the key to understanding Egyptian hieroglyphs—a script made up of small pictures that were used originally in ancient Egypt for religious texts. Hieroglyphic writing died out in Egypt in the fourth century C.E. Over time, the knowledge of how to read hieroglyphs was lost until the discovery of the Rosetta Stone in 1799 and its subsequent decipherment. The Stone is a tablet of black rock called granodiorite. It is part of a larger inscribed stone that would have stood some 6.5 feet high.

Source: Khan Academy