Blue Milk Glass? What is it?

I picked this up at a local antique shop yesterday because I recently became interested in learning more about both milk glass and depression era glass. This piece was marked 'Blue Milk Glass...Do you know what it is?" I thought it would be fun to investigate what this might be - but I've searched for all the things I thought it 'could' be, and I'm coming up empty handed. If anyone can tell me more about this piece, I'd appreciate it!! It has substantial weight to it, and the underside is concave. I thought it might be a mold of some sort: butter, cookie, or dough; but now I'm wondering if perhaps it is the lid to some type of glass container? The color appears to be a creamy powder or baby blue, with striations visible if I look close enough. Please help me figure out what this is! :-)

Additionally, the measurements of this piece are as follows: It's 3 and 1/2 inches tall (top to bottom). The top octagon shaped portion of this piece measures 2 and 1/4 inches wide (side to side). Lastly, the bottom round section measures 2 and 1/2 inches wide (side to side), with the outside diameter measuring 8 inches around. The opening on the underside of this piece (where it is concave) has an opening measuring approx. 1 and 7/8 inches wide. Looking at the piece straight on, the bottom round portion measures 1 inch tall.