The Crystal Balls were Pieces of Eden that allowed their wielders access to the Nexus, a place where individuals could speak across centuries and where members of the First Civilization communicated with people across the timeline. The Crystal Balls were round in shape and possessed lines and size similar to the Apples of Eden.
The first known Crystal Ball was in the possession of the English scientist Edward Kelley, who used it during his time in the Austrian royal court to gain insight from the First Civilization. He used it in his research to develop new advances in technology, becoming the personal scientist of the Austrian ruler in the process. However, the Ball was taken by Kelley's brother to keep him from abusing its power.
A second Crystal Ball was possessed by the Kanien'kehá:ka in Kanatahséton. This one was used by Ratonhnhaké:ton to enter the Nexus, where he communicated with Juno. She directed him to become a member of the Assassin Order, with the secret intent of allowing him to come into the possession of the key to the Grand Temple. Ratonhnhaké:ton later used the ball again, and Juno then told him to hide the key somewhere where nobody would find it, while she knew that Ratonhnhaké:ton's descendant Desmond Miles would later find it and unlock the door inside the Grand Temple. After this final use, the artifact disintegrated into dust.