Wondrous item, rare (requires attunement)
This crystal ball is about 6 inches in diameter. While touching it, you can cast the arcane eye spell with it. The spell ends early if a creature touches the magical eye; the creature then knows that it had been observed by divination magic. For the duration of the spell, you are blinded, and the crystal mimics the appearance of one of your eyes.
 
It's astonishing what people do when they think that no one's watching.