If it's really sharp it's obsidian.
I think a uranium dagger would be pretty cool, and pretty deadly.
Obsidian is sharp, but it's still glass.
It's really brittle and would be terrible as a dagger and difficult to craft into an actual weapon.
And you'd only likely find it in areas of volcanic activity or previous (as in ancient) volcanic activity.
Unless OP is in an area like that, I wouldn't expect it to be Obsidian.
Do you have a local university or college nearby?
Or even any sort of scientific research centre.
You could go and bring a sample down and try to find someone who knows what it is or ask if they could find out what it is.
But, don't go telling them that you plan to turn it into a dagger. They'd probably take that in the wrong way.
