The Mermaid's Scale is a quest item from The Legend of Zelda: Link's Awakening. Part of the trading quest for the Magnifying Lens, it can be obtained from Martha the mermaid, who has promised a scale from her tail to anyone who brings her Necklace back. When Link recovers her the necklace, she allows him to take a scale. This scale in turn can be used to complete The Mourning Mermaid, a sculpture of Martha carved by Schule Donavitch and placed on the southern shore of the bay, this allows Link obtain the Magnifying Lens in the cave that the statue is sealing off by moving it.
Interestingly, after Link receives the Mermaid's Scale, Martha reveals that the alligator artist, Schule Donavitch, was also looking for a scale.