From Pikipedia, the Pikmin wiki
The Joyless Jewel (ぬかよろこびジュエル?, lit.: "Premature Joy Jewel") is a treasure in Pikmin 2. It is found in the seventh sublevel of Hole of Heroes, inside the body of a Ranging Bloyster. Before getting the treasure from the Bloyster, defeat the four Armored Cannon Beetle Larvae by letting them fire boulders at one another. Divide the leaders so the Bloyster can't eat and follow one of them. After all of the enemies are gone, cash in the treasure for 100 Pokos. The Joyless Jewel is actually a plastic, silver ring with a green heart.
“We found this treasure in a cave teeming with merciless creatures. But we did find this gorgeous jewel, so it wasn't all bad. Update: It turns out that it's just plastic. So it was all bad after all.”
“Those who view this gem's green twinkle become filled with great hopes and expectations. However, it rewards neither. This artistic sham only provides short-lived happiness.”
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