Blue Beach Stones....Is it Slag?

I visited my favorite Lake Erie beach twice this week.'s WORK, I tell you!
I have to keep a fresh supply of beach stones and beach glass on hand for SToNZ, you know.

So this week I found a great deal of this gorgeous blue stuff. It's really beautiful....especially when wet. You can rub it with mineral oil to deepen the color, or leave it dry for a lighter more subtle blue. It's great stuff.

I have been collecting it for several years. I'm thinking that maybe it's time to put it all together and sell it in my Etsy shop as a collection. It looks gorgeous in a bowl....or can be used in jewelry making.

I am thinking it might be slag glass of some sort. Probably tossed in the lake years and years ago by iron manufacturers on its shore. If anyone out there has an idea of what this is, do let me know!  I think I'll list it in my Etsy shop. Pretty, isn't it!?