The Rosetta stone is very famous for it provided the key to solve the ancient Egyptian language. The Rosetta Stone was carved in 196 B.C.
It was discovered by the French soldiers who came with Napoleon. The Frenchman Jean Fran殩s Champollion is the one who cracked the code of the stone.

The discovery of the stone of rosette later called Rosetta stone is an interesting story. The stone was discovered by the French troops in Napoleons military expedition, in 1799 in Lower Egypt, when they were digging the foundations of an addition to a fort near the town of el-Rashid (Rosetta), in the Nile Delta. It was discovered near the town of Rosetta (now Rashid), located in the Nile Delta about 40 miles northeast of Alexandria, by a Frenchman, Pierre Bouchard, on 15 July 1799. Captain Bouchard, an engineer officer in Napoleons expedition to Egypt, was supervising the reconstruction of an old fort, as part of the preparations for defending the French from attacks by British and Turkish forces in the area. The Rosetta Stone came to light during the demolition of a wall in the fort. Captain Bouchard saw that the polished black basalt stone contained three sections of different types of writing, and recognized its significance immediately. He sent the stone to Cairo, to the scholars who also accompanied the French expedition to Egypt.
In 1801, after two months of warding off attacks by the British, and after their defeat at Abuquir Bay, the French forces in Egypt surrendered. Under the terms of the Treaty of Capitulation, all antiquities in the possession of the French, including the Rosetta Stone, were ceded to the British.

The stone is a compact basalt slab (114x72x28 cm) that was found in July 1799 in the small Egyptian village Rosette (Raschid), which is located in the western delta of the Nile. The stone contained words in three types of writing: Egyptian hieroglyphs, Demotic, which is a shorthand version of Egyptian hieroglyphic writing, and Greek. By translating the Greek section, scholars were able to learn what the hieroglyphs meant. This enabled them to translate inscriptions inside the Egyptian temples.

The inscription on the stone was a decree passed by a general council of priests which assembled at Memphis on the first anniversary of the coronation of Ptolemy V Epiphanes, king of all Egypt. The writing concerns the honours bestowed on the king by temples of Egypt in return for services rendered by him to Egypt both at home and abroad. Priestly privileges, especially those of an economic nature, are listed in detail. Because the inscription appears in three scripts, hieroglyphic, demotic, and Greek, scholars were able to decipher the Egyptian hieroglyphic and demotic versions by comparing them with the Greek version.
The representation of a single text of the three mentioned program variants enabled the French scholar Jean Francois Champollion (1790-1832) in 1822 to basically decipher the hieroglyphs. Furthermore, with the aid of the Coptic language (language of the Christian descendants of the ancient Egyptians), he succeeded to realize the phonetic assessment of worth of the hieroglyphs. This proved the fact that hieroglyphs do not have only symbolic meaning, but that they also served as a "spoken language".
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Another British physicist Thomas Young worked on the translation of the stone with the French Egyptologist Jean Fran殩s Champollion.
Thomas Young, the English Physicist, was the first to prove that the elongated ovals or cartouches in the hieroglyphic section of the stone contained a royal name written phonetically, in this case that of Ptolemy. Jean Fran殩s Champollion went on to correct and enlarge Youngs list of phonetic hieroglyphs and lay the foundations of our information of the ancient Egyptian language in a paper which was read to the Acad謩e des Inscriptions et Belles-Lettres in Paris in 1822.
It was this discovery -- that the Egyptian hieroglyphic writing system used a combination of ideograms, phonetic signs, and determinatives -- that provided the breakthrough in the translation of hieroglyphic writing. And this ability to read the ancient hieroglyphs in turn opened the door to the history of ancient Egypt and gave birth to the new discipline of Egyptology.