The famous Rosetta Stone, discovered in 1799, is a large carved stone slab of black rock, called granodiorite, covered in texts.
The texts are a decree passed by a council of priests and King Ptolemy V, meant for public display, as part of a larger inscribed stone that would have stood some 2 meters high.
Fortunately, the same texts decree, attesting to King Ptolemy V’s generosity and devoutness, was inscribed in three ancient languages: hieroglyphics, demotic and ancient Greek.
And because scholars could read the Greek transcription, the stone became the key to deciphering hieroglyphics, and hence ancient 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.
The Stone dominates Room 4 at the British Museum because of its very importance in deciphering ancient history particularly, the ancient Egyptian history, language, literature, religion, architecture, and art.
The Rosetta Stone, seen by millions of visitors yearly, has been exhibited in the British Museum mainly since 1802.
Source: History and Khanacademy