Odin's Gem is a mineral that is first seen in the episode "Treasure Riders".
The Rescue Riders, Marena, and Waldondo del Mundo went on a treasure hunt to find Odin's Gem. However, it turned out to be a trap set by Waldondo, who planned on capturing the Rescue Riders. After saving them, Marena revealed that she was already in the possession of Odin's Gem, which she kept in her belt. She tossed the gem to Burple, who swallowed it and hid it in one of his stomachs.
Odin's Gem resembles a real life diamond, and may even be one. It is quite big for a precious gem, about the size of an adult human's hand.