You actually have part of an insulator. Your piece was part of a 2- or 3- part insulator and there would have been an additional shell or two cemented to the top portion where it's not glazed. It's almost impossible to tell exactly which insulator it would have been a part of. Here's a photo from an old Locke catalog to give you an idea of what a two-part insulator would look like.
As far as age, that blue-gray color is very unusual and whole insulators in that color are highly sought after. That glaze color dates it to the early 1900s, probably in the 1910-1920 range, give or take a couple years.
Value.... well, it's tough to say. Most collectors what a whole piece and there isn't much demand for one shell, even though this one is old and unusual.
As a side note, I dug up a broken three-part insulator in a similar blue-gray color some years ago - here's a photo. If it was in mint condition, it's probably a $300+ piece. It's pretty broken up so it really doesn't have much value beyond being a souvenir of my digging in an insulator factory dump.