Which powerful jewel is Knuckles sworn to guard in the Sonic series?

Knuckles is sworn to guard the Master Emerald in the Sonic series.

The Master Emerald is a large gemstone associated with Angel Island, and Knuckles serves as its guardian. In the series' mythology, the jewel has the power to control or neutralize the Chaos Emeralds, making its protection central to Knuckles's identity and responsibilities.

Knuckles first appeared in Sonic the Hedgehog 3, where Dr. Robotnik deceived him into believing that Sonic wanted to steal the Master Emerald. The jewel becomes the focus of Sonic & Knuckles, in which Robotnik steals it and Knuckles pursues him across Angel Island. The story establishes Knuckles as a protector rather than simply another rival.

The Master Emerald is distinct from the seven Chaos Emeralds, which characters commonly collect to gain powerful transformations such as Super Sonic. Later stories often show the Master Emerald being shattered into pieces, but those fragments are still parts of the same artifact. The Phantom Ruby and Time Stones belong to different Sonic storylines and are not Knuckles's traditional charge.