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A Mastery Gem is an item found in Wario: Master of Disguise. They are similar to Guise Gems and are obtained in the same way, by opening a green treasure chest and winning a game of Color Me Happy. However, instead of giving Wario a new disguise, they upgrade one of the disguises Wario already has, giving it new powers.
The Sparky Mastery Gem is the only one that is not necessary to finish the game.
List of Mastery Gems
There are a total of seven Mastery Gems, one for each disguise barring Thief Wario, which is never upgraded.