I've had this turquoise rock/mineral/man made? sitting on my bookshelf for about two years that I thought was just slag until I looked at it again and realized that it just wasn't.
I did find it in a creek that contains a lot of blue slag, but I have a lot of blue slag and have a lot of experience with blue slag, and this isn't blue slag.
It's waxy. Sandpaper can get dust off of it, but it's very hard. It will not break, like slag glass will. It's solid. It has no air bubbles or folds in it like my other slag collections. It seriously looks like a speckled robin egg. It makes a very light blue/white streak on sand paper. Oh and I found it in Pennsylvania in a creek.
The Rock itself
The blue slag you usually get out of the creek color and air pockets
The glossiness compared to a piece of slag.
I was able to collect the dust from the sand paper and look at it under a very crappy microscope (sorry)