So, here are the pics. It was really hard to photograph so this is as good as it'll get. I don't think it's a sulphide but it is a very strange marble so if you have any ideas as to what it is and any guesses as to what's inside i'd love to hear it!
It isn't that green. I tried to fix it but I couldn't get it to look any better. I didn't think of a tadpole, but it does. I was thinking a flower bud, but I don't know.
That's a machine made marble, not a sulphide. The internal oddity is probably just a spurt of color. It could be that the tank with color in it ran out or had a bit of air trapped or a bit of oven brick clogging it and only a little color came out, so this was all it got. It could also just be a foreign object like a bit of oven brick (happens fairly often).