I'm going to say this. When a stone has been cut and polished or ground down, there is little chance of determining its identity from a picture alone. I will try to help. It does not look like obsidian, which is black, but it would look like glass. Shine a light behind it. Does it block the light completely or is it translucent? Is it magnetic? Finally, your best option is to see if the person who sold it to you or the shop where it was sold knows anything about it. Another option is to ask a jeweler.
Your guesses could help too.