A golden human skull, with a small slot on the top of its cranium. Drop a gold coin in the slot (up to 3 times per day), ask a yes or no question, and get an answer. The PCs figured it was just using a simple commune spell, and they didn't give it a lot of use during the month or two they had it.
When they took it to the big city and had it officially identified (no PC had the spell), word of it got around to the Thieves' Guild. The TG then paid the PCs for the opportunity to ask the skull some questions. Under the supervision of the PCs, two thieves, posing as nobles, asked it some specific but seemingly mundane questions. "Did the widow Sophie kill her husband?" "Is my son in his room at our home?" Etc. The answers meant nothing to the PCs, but the TG realized the power of the item.
The skull answers with absolute accuracy, regardless of divination-interfering spells. For instance, the thieve's son sitting in his room at home was under several screen-type spells to prevent divinations and scryings, and there was an illusion of him elsewhere in public. But the gold skull answered correctly.
The skull would give one of four answers: Yes, No, Yes and No, Unknowable. It could only give factual-based answers. It could not predict the future ("unknowable"), but its knowledge of facts spanning all the world and all the universe and planes was perfect. To an adventuring party, the skull was not particularly powerful. But to a thieves' guild or a king's court or a sage guild, it was priceless. The TG was most interested in just keeping it out of the king's court. The PCs sold it in the end during a "campaign upheaval". Now the TG has it buried away in its secret vaults.
Quasqueton