Big glass button

I found this in my grandmothers sewing notions, so have no idea of it's age. It's fairly large for a button and I took the picture on a large, standard size keyboard so you would get an idea of size next to the keys.

The image is on a green glass button, which is slumped in the center. Looking at the back, it appears that this is intentional. The hole is made out of glass (as opposed to metal, like some other painted buttons I've recently run across).

When I look at the image under magnification, I see the tiny dots so I know it's not hand painted. I'd still appreciate any information about it.