From the Super Mario Wiki
Jewelry Land is a peaceful kingdom ruled by King Fret and Prince Pine, and is the setting of the game Yoshi's Safari.
The kingdom gets its name from the magical gems that maintain peace and stability across the land. When Bowser and his Koopalings steal the gems, a massive earthquake splits the land into a Light Realm and a Dark Realm. Bowser takes control of the Dark Realm, holding Prince Pine in captivity, while his kids run riot in the Light Realm with King Fret as their prisoner.
Princess Peach hears about the invasion and asks Mario and Yoshi to help save her friends. Armed with a Super Scope, Mario and Yoshi set off to take down Bowser's armies and restore peace to Jewelry Land.
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