A giant dino egg... The entire shell is still intact!
The Dinosaur Egg is an artifact that can be found by digging in the Mountain,treasure chest, while fishing or while tilling the ground. They can also be obtained from a Dinosaur. They can be sold for 350g (No star), 437g (Silver star), or 525g (Gold star).
Other than selling or placing in the Museum, you can place the egg into the Incubator inside a Big Coop. It will hatch a dinosaur and lay a Dinosaur Egg (Artifact, the same as one that dinosaurs hatch from).
Tip: Don't turn it into mayo, as it will be the same price as regular mayo (100g/225g (gold star)).