Emerald of the East
From the Super Mario Wiki
The Emerald of the East is an artifact seen in the Saturday Supercade episode Mississippi Madness. It is the gem displayed in the museum in the episode, as well as what Belle, Billy Bob, and Colonel Culpepper planned to steal.
When the trio befriended Donkey Kong, they sent him to steal the emerald. Donkey Kong managed to do so, even while kidnapping Pauline and fending off Mario. Donkey Kong held on to the emerald during his retreat to the jungle, but was found by Mario almost right away who proceeded with his trap. When Belle, Billy Bob, and Colonel gave chase, crashing into Donkey Kong just as Mario was about to trap him; doing so caused Donkey Kong to drop the emerald right into Colonel's hand but also trapped the trio in the dropped cage. It is unknown what happened to the emerald after the episode, though presumably it was returned to its rightful location.