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A Guise Gem is an item found in Wario: Master of Disguise. They are found in green treasure chests. When one is obtained, Goodstyle will eat it, giving Wario the ability to wear a new disguise. Acquiring the gem requires winning a game of Color Me Happy. Mastery Gems are similar to Guise Gems, but give new powers to a disguise Wario already has.
List of Guise Gems
There are a total of seven Guise Gems, one for each disguise barring Thief Wario, which is available from the start.