Taydenite

Taedenite the rarest living gem in the galaxy. The villian Vulkanus is known to deal in this rare gem which can be used to power starships. It first appears in the episode "Kevin's Big Score". When Kevin arrives looking to obtain an important item (a holo-disc from Grandpa Max), Vulkanus decides to get back at him for a previous betrayal by forcing Kevin to absorb a small piece of Taedenite, thus causing Kevin's body to mass-produce the gem, thus giving Vulkanus an endless supply. However, he is foiled by Ben & Gwen who arrive to save Kevin. Rather than attempt to arrest him, however, Kevin leaves Vulkanus with the small fortune in crystals he had managed to chip off beforehand, considering it as payment for his past actions.

Taedenite is revealed to be able to power starship in the episode "Con of Rath". According to Gwen Tennyson (in the episode "Con of Rath"), every time she, Kevin, and/or Ben come accross it they run into Vulkanus, which is proven true when Vulkanus appears shortly afterwards (This maybe due to the gem rarity and value). During the episode, Vulkanus and his henchmen are shown have a small Taedenite mining operation, which is destroyed by Ben (as the Omnitrix Alien, Rath).