From a box of old Christmas ornaments, an early glass acorn Christmas tree ornament in two shades of blue. More than likely a German mercury glass decoration, dating from the 1920's or perhaps earlier.
Nicely molded detail of the ribbed sides and bumpy top. 2" in height from the bottom to top of cap. Unmarked original metal cap with loop. Cap fits tightly over the glass neck of ornament.
Wear to the paint, as you note in photos. Aside, no other damage that I found.
Add this little fruit nut to your feather tree collection for fun conversation.