Went to the jewelry supply store a couple of days ago with my youngest son Jarrett and let him pic a stone for that ring I found last week on the beach. He chose a black obsidian cabochon which had to be sized to fit the ring. I got the black cabochon stone back a couple of days ago and put it on the ring tonite. My son picked a well suited stone for the ring. 
Beauty of a ring. I've got a big hunk of obsidian around here somewhere. Used to use it as a door stop. But it would have been a bit heavy for your ring.