Which is the dinosaur egg?
The specimen on the lower right is an approximately 80 million-year-old fossil dinosaur egg from the Late Cretaceous Djadochta Formation of Shahbarakh Usu, Mongolia. It was collected by A. F. Johnson on 17 July 1923 as one of a group of 3 weathered oviraptorid eggs.
The object on the upper left is a water-worn rock, most likely from a river. Its resemblance to an egg is merely accidental.
River-rounded rocks are commonly mistaken for fossil eggs. This is just one method by which nature produces rocks that resemble eggs. Sedimentary concretions are another common imposter. Concretions often form when some object acts as a “seed” for the deposition and cementation of sequential layers of sediment. On occasion, the matter that initiates the concretion, the “seed,” can be a fossil. To find this out requires cracking or cutting open the concretion.
Genuine fossil eggs usually have an easily identifiable shell which differs significantly from the enclosed sediments either by having a fine surface ornamentation (the smoother the "shell," the less likely it is to be a non-bird dinosaur egg) or a specific type of crystalline structure in cross-section. Also, because eggshell tends to be brittle, the shell is almost always heavily cracked with clear shifting of the eggshell bits. Ironically, one strong indication against a dinosaur egg identification is a very egg-shaped specimen: most fossil eggs are not "egg-shaped" because most fossil eggs come from non-avian dinosaurs and are everything from spherical to torpedo-shaped.
Additionally, the thickness of the “shell” can rule out an egg ID. Embryos in hard-shelled terrestrial eggs need shells through which they can conduct gas exchange – basically so they can breathe. Past a certain thickness, this becomes impossible.
And don’t forget, if a chicken egg were included in the photo, it, like the oviraptorid egg would be properly identified as a dinosaur egg.