Green Gecko Gem
From the Super Mario Wiki
The Green Gecko Gem is a crystalline artifact seen in the Nintendo Comics System comic Piranha-Round Sue. The gem gives whoever holds it an impenetrable forcefield, although "the strongest enemies" can get through it. This seems to apply to the likes of Bowser Koopa, as he overpowered its user with no trouble.
Originally in possession of the Mushroom King, the gem is given to Mario and Toad when they are assigned to collect a magic wand to transform the king from a humanoid lizard to his normal form. However, some time after finding the wand, Mario loses both it and the Green Gecko Gem to Piranha Sue when he tries to save Toad. She then claims to rule the world when Bowser appears and chokes her, then collects the items; however, he tosses the Green Gecko Gem away, claiming it to be a "cheesy trinket," and finds the wand to be a fake.