slag glass - see the conchoidal fracture at the bottom by the shine from the light? Obsidian doesn't come in this color, and jasper doesn't look this glass-like. It can still be cabbed, or used in a fish tank or on a window sill - still a beautiful find! Keep lookin' down!
why do you think its a rock?
based on you pic, looks like glass or pottery,
the lines almost look like brush marks
the lines don't look right for a rock
Just an opinion
need to see more pics of the other sides to make a good ID
Brady