A Mysterious Egg is a rare item found by exploring certain locations. A tortoise egg (brown speckled) is only found in an elephant graveyard, while a raptor egg (blue spots) is only found in a raptor nest.
An egg can be traded during an expedition, but cannot be exchanged for fame or funds. If kept in the player's inventory for a few days, the egg will hatch into a baby tortoise or baby raptor.
The secret explorer Sound of Mystery starts out with both kinds of mysterious eggs.