Of all the different button materials, I have to say glass is my favorite. Black glass buttons are supposed to be "common", but not so. This example has a flower with a silver lustered center and leaves. As always, I would loved to have seen the garment this was sewn to. Whatever the history, it would make a wonderful addition to someone's collection, or a special touch to your handmade item.
In good condition, let the pictures be your guide. Measures 3/4 in across.