This is about the actual ring. For the book, see The Ring of Solomon.
"A few years previously, Jerusalem had been a dowdy little town, not especially notable, and certainly not to be compared with capitals such as Nimrud, Babylon, or Thebes. Now, it vied with those ancient cities as a place of wealth and splendor-- and the reason for that wasn't hard to guess. It was all about the Ring."

The Ring is a powerful artifact used by Solomon during his reign as king. It contains and draws power from the Greater Spirit Uraziel.

It is a gateway to the Other Place, which makes it very powerful. Unfortunately, because it is a gateway, it is painful for either humans or djinni to use as if their spirit is being torn apart, pulled toward the Other Place. Bartimaeus describes it as akin to a dismissal.

Touching the Ring will summon a great host of spirits, including marids and afrits (this comes from the nature of the Ring as a gateway). Turning the Ring on the finger of the user summons the spirit of the Ring, Uraziel, who does the bidding of the person who wears the Ring.

The Ring was created by a civilization near the Tigris-Euphrates. The civilization was extinct by the time of Solomon.