My uncle sent me this item... he thought it might be an insulator. I'm pretty sure it is not, but it begs the question... alright then, so what is it? It was found in an old house. It is solid glass, with short external threads and what seems to be a rubber washer. So I guess it was meant to screw into something, with the washer sealing tightly? There is a shallow cone-shaped "pinhole" at the base, but it isn't very deep. Corrugated base, no markings. Dimensions are 4" high by 2 1/2" wide. Additional photos follow.
Update: The best suggestion so far is that it might be some sort of light cover or diffuser. If anyone has a more definite idea, let me know and I will update this description.