It's the Mermaid's heart that K'nuckles stole from the episode Gone Wishin'.
It is a valentine-shaped orb made of golden glass. It isn't really a mermaid's heart but a symbol. According to K'nuckles its a heart filled with magic, wish-granting mermaid tears. When eaten a wish will be granted,and when all the tears are gone the Mermaid dies and gets turned in a Skymaid.