What a charming and curious thing! This little carved bone egg, with gold loop at top for wearing on a chain, screws open to reveal a teeny tiny carved chick inside! Since finding this one, I've found a few others online like it, and learned that they were a bit of a thing during the Victorian era--some plain like this one, others painted on the exterior, as well as a few with little windows carved in the side of the egg to allow a peek at the chick inside. Not surprisingly, I've also learned that it is fairly rare to find them still with the chicks inside! I'd wear it as the pendant on a necklace (this egg screws tight to keep the chick secure inside), surely bringing good luck.
These photos are shot at magnification, showing the irregularities in the bone more dramatically than they appear to the eye; this piece is in very good condition, with one dark area on one side of the exterior, which I assume a discoloration in the bone though perhaps is the product of age/wear. 3/4"long x 1/2".wide.