"Stone of Destiny" sound so cool. Here's Wikipedia:
Traditionally, it is supposed to be the pillow stone said to have been used by the Biblical Jacob. According to one legend, it was the Coronation Stone of the early Dál Riata Gaels when they lived in Ireland, which they brought with them when settling Caledonia. Another legend holds that the stone was actually the travelling altar used by St Columba in his missionary activities throughout what is now Scotland. Certainly, since the time of Kenneth Mac Alpin, the first King of Scots, at around 847, Scottish monarchs were seated upon the stone during their coronation ceremony. At this time the stone was situated at Scone, a few miles north of Perth.
In 1296 the Stone was captured by Edward I as spoils of war and taken to Westminster Abbey, where it was fitted into a wooden chair, known as St. Edward's Chair, on which all subsequent English sovereigns except Queen Mary II have been crowned. Doubtless by this he intended to symbolize his claim to be "Lord Paramount" of Scotland with right to oversee its King.
Some doubt exists over the stone captured by Edward I. The Westminster Stone theory posits that the monks at Scone Palace hid the real stone in the River Tay or buried it on Dunsinane Hill, and that the English troops were fooled into taking a substitute. Some proponents of the theory claim that historic descriptions of the stone do not match the present stone. If the monks did hide the stone, they hid it well; no other stone fitting its description has ever been found.
In 1328, in the peace talks between the Kingdom of Scotland and the Kingdom of England, Edward III is said to have agreed to return the captured Stone to Scotland. However, this did not form part of the Treaty of Northampton. The Stone was to remain in England for another six centuries. In the course of time James VI of Scotland came to the English throne as James I of England but the stone remained in London; for the next century, the Stuart Kings and Queens of Scotland once again sat on the stone — but at their coronation as Kings and Queens of England. Since the Act of Union 1707, the coronation ceremony at Westminster Abbey has applied to the whole of Great Britain, and since the Act of Union 1801 to the United Kingdom, so the stone may be said to have returned, once again, to its ancient use.
And here's an article from the Telegraph saying it might be a fake. Via.