Someone asked me what this is. Of course anything "unknown" made of glass must be an insulator! I have an exact (or extremely similar) one. Mine is SCA, likely from manganese in the glass. I was thinking it might be a sidewalk vault light?
It's about 5" tall and 2" square at the bottom. Note the semicircle vertical groove on the right of the second picture.
UPDATE: This is believed to be a furniture leg. They come in several styles and colors.