From the Super Mario Wiki
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The object is to catch gems such as rubies, emeralds, sapphires, and diamonds. Diamonds are special because the player haves to catch them by using a home run buddy catch, unlike the other gems. Although simple in concept, there is a catch: items are being thrown at the player while trying to catch the gem, and if the player gets hit, they fail the catch. The Shy Guy in this minigame that helps players with buddy jumps shows good chemistry with all characters, not just the characters Shy Guy normally shows good chemistry with. If the player uses Red Shy Guy, the Shy Guy that helps with buddy jumps is colored blue instead.
This minigame only takes place in daytime, not at night.