It was indeed a sad ending. From what I can recall, gem mermaids were secretive creatures who never interfered with anyone on land. But one gentle gem mermaid saved a Cookie from drowning, and they became very close. As a promise of their affection, she gave him her most precious treasure... a priceless oyster pearl.

However, the Cookie was a greedy soul, and instead of valuing what he gave her, he sold the pearl, and broke her heart.