Classic late Victorian black glass button, dates late 1800s-early 1900s. Most were made in Czechoslovakia. You should be able to find your exact button in the standard books on these, which I used to own but no longer. Queen Victoria set a fashion for these that lasted until the turn of the 20th c.
They have very little value in general, $2-5 typically. Vast quantities of these entered the market since discoveries were supposedly made in Czechoslovakia of originals. In fact many believe the new surge of material were actually made in modern times from period molds.
Here's a nice informational website. Google Victorian black glass buttons and you'll find scrillions of button sellers pushing these. If you can ID your precise button, it will be a number from that book, you can search and see if you got lucky and perhaps it is an uncommon one... probably not.
http://www.beadandbutton.com/bnb/default.aspx?c=a&id=3473