It is a large acorn in the shape of a heart. It has a wooden arrow through its center.
Role in series
When SpongeBob walks to Sandy's treedome on Valentine's Day, Sandy gives the acorn to him, making a joke to complement the gift. SpongeBob puts it in his pocket before giving Sandy a chocolate syrup heart.
SpongeBob's Truth or Square
It is a happiness object in the second level, "Meeting Sandy Cheeks." If the player completes a tiki challenge and picks the acorn up, it is added to their object collection. In the Xbox 360 version, finding the heart-shaped acorn will give the player the "Valentine's Day" achievement.