The Jagged Crown is a symbol of office for the High King of Skyrim, a symbol of the might and power of the ruler of the Nords going back to the time of High King Harald (1E 143-221), or even before. Legend holds that it was forged from the bones and teeth of ancient dragons by Harald himself. A portion of the power of every king who has worn it is said to be contained within.
Jewel of Fire
Legend has it that the Jewel of Fire is a star fallen from the sky, for it glows with an inner flame. The Jewel was used by Jagar Tharn to store the power of the Staff of Chaos, so that if anyone tried to recreate the staff it would be useless. The Jewel played a large role in the Imperial Simulacrum.
In 3E 389, Jagar Tharn stole the Staff of Chaos from Barenziah's court, and used it to imprison Emperor Uriel Septim VII in the realm of Oblivion, also killing his apprentice Ria Silmane so the Elder Council would not learn of his betrayal. Tharn wanted to make sure no one could release the emperor, so he attempted to destroy the staff. However, since the Staff of Chaos is indestructible, it was instead split into eight pieces, which Tharn then hid in forgotten, dangerous places throughout Tamriel. Tharn then made sure that no one could ever recreate the staff by draining its magical power into the Jewel of Fire, which he kept hidden in the Imperial Palace.