I came across two to of these today, and they look like insulators to me, but I'm not sure. They are definitely glass. There is a iron collar around the glass that would have been screwed/attached to something. There is a hole that goes completely through the piece and a deep grove on top. It's approx. 2 1/4" tall. Any help greatly appreciated!

ANSWERED: Two people have informed me they are lighting rod insulators, thanks!