The Knowing Jewel (「知玉」?) is an artifact in Ōkamiden, being kept in the Sage Shrine and guarded by Shikibu. It is said to know all things, and vast streams of information can be seen flowing through it. This jewel was used to reveal information about Akuro.
Its appearance is similar to various structures in the Tao Troopers' headquarters building in Sei'an City. It also gave a small reference to Waka through a vision that Chibiterasu had while looking at it.
Judging by the tiny size of the Sage Shrine, the Knowing Jewel has to be the size of a small marble.