An important rock
Just a block of stone. Yet on it lay the clue to breaking a code that baffled scholars for centuries: Egyptian hieroglyphs.
This fragment was found at el-Rashid (Rosetta) in Egypt , hence its modern name: the Rosetta Stone.
It was dug up in 1799 by French soldiers of Napoleon Bonaparte, who were enlarging a fort.
After the British defeated the French, it was handed to the British Army who sent the Stone to the British Museum, where it's been since 1802 (apart from a few trips such as two years stored underground during the First World War).
Before the Stone was found, no one could read ancient writing on Egyptian monuments, tombs and statues, which looks as if it is a form of picture-writing.
The secret of reading hieroglyphs had been lost by the end of the AD 400s.
By a stroke of luck, the Rosetta Stone held the key because of when it was carved. It was made in 196 BC, when Egypt had a new boy-king, Ptolemy V, whose family was originally Greek.
On the stone was written an official announcement of gifts made by the king to the Egyptian temples.
Priests used the traditional form of writing we call hieroglyphs, but on the Rosetta Stone the same announcement was written in two other languages: everyday Egyptian and Greek.
This was a breakthrough for the code-breakers.
As he studied the Rosetta Stone, English scientist Thomas Young realized that some loops, called cartouches, surrounded the name of a king - which he knew from the Greek writing was Ptolemy - and so the hieroglyphs inside the cartouche must write the sounds in his name.
In the 1820s, a French scholar, Jean-François Champollion, finally 'translated' the Stone; using the Greek and other inscriptions realised that not only was the name written with signs representing sounds, but so was all of the announcement on the Stone.
With the code broken, and the hieroglyphic 'alphabet' revealed, people could read other Ancient Egyptian writings, and learn far more about life in Ancient Egypt.
The Stone itself is part of a slab of grano-diorite (a volcanic rock), roughly 114 cm high and 72 cm across.
It originally stood in a temple at Sais.
The top part of the slab was probably broken off long before the Stone was moved to Rosetta and has never been found.