Imperial Regalia

The Imperial Regalia (神器, Jingi; literally "Sacred Treasures") are three mystical and mysterious items that were given to the King of Souls by the Kami themselves, having been said to have been placed in his care prior to the very founding of Soul Society itself. They are said to be the key to the vast scope of abilities that he is capable of preforming.

Yasakani no Magatama
The Yasakani no Magatama (八尺瓊曲玉, literally "Jewel of Eight Scales") is a necklace of several small bluish-purple sphere-shaped jewels, composed entirely of what Sōsuke Aizen and Kisuke Urahara both acknowledged as the Hōgyoku. They carry the power to quite literally dissolve the barries between two or more states, regardless of its properties. The wall between Shinigami and Hollows, the wall between worlds, the wall between the known and the unknown, and the wall between mortals and the divine... can all be torn asunder by the power of these jewels, among others. They appear to be linked to the King of Souls directly. It is suggested that Aizen had caught a glimpse of the King himself through his unison with the Hōgyoku, due to the connection the jewel itself has with the King of Souls. Aizen noted that fusing, or rather "subjugating", the Hōgyoku would allow him to transcend his dimensional existence. In doing so, he took on a radical transformation that caused him to assume a form that resembled the King's own appearance. Ideally, he would essentially obtain a "fragment of the King's power".

While the jewel beads themselves have their own purpose, the main use of them is that the King uses them as raw materials for various items that each possess a significant degree of power and ability scope, allowing for a range of possibilities. Such items created by the King include:
 * Ōken: The Hōgyoku jewels are used primarily for the purpose of imbuing the members of the Royal Guard with the King's Key, allowing them to enter the Reiōkyū dimension at will. It acts as a "seed" of sorts which is placed within their Saketsu, or "Soul Sleep", which, when combined with a great amount of absorbed souls, results in producing an Ōken that is literally infused into the body; more specifically, into their bones.
 * Ōin: While the process of how it occured is currently unknown, the King has confirmed that the Ōin itself was born from a Hōgyoku jewel.

Behind the Scenes
This article was obviously inspired by the Imperial Regalia of Japan, although the author has taken several liberties in order to effectively introduce the concept into the Bleach universe.